Especially description regarding the build process and `build` directory
structures, along with some typo fixes and cosmetic changes.

Fix heap-buffer-overflow for small `Hash` (HT) in `Hash#rehash`

- Move `#catch` definition to `mruby-catch.c` to avoid tweaking
- Remove `#__preserve_catch_method`
- Implement whole `#throw` method in C

### Example

##### example.rb

```ruby
h = {}
(1..17).each{h[_1] = _1}
(1..16).each{h.delete(_1)}
h.rehash
```

##### ASAN report

```console
$ bin/mruby example.rb
==52587==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000006998 at pc 0x55a29cddf96b bp 0x7fff7b1b1720 sp 0x7fff7b1b1710
READ of size 4 at 0x602000006998 thread T0
    #0 0x55a29cddf96a in ib_it_next /mruby/src/hash.c:639
    #1 0x55a29cde2ca2 in ht_rehash /mruby/src/hash.c:900
    #2 0x55a29cde379f in h_rehash /mruby/src/hash.c:996
    #3 0x55a29cde7f3d in mrb_hash_rehash /mruby/src/hash.c:1735
    #4 0x55a29ce77b62 in mrb_vm_exec /mruby/src/vm.c:1451
    #5 0x55a29ce5fa88 in mrb_vm_run /mruby/src/vm.c:981
    #6 0x55a29ceb87e1 in mrb_top_run /mruby/src/vm.c:2874
    #7 0x55a29cf36bdf in mrb_load_exec mrbgems/mruby-compiler/core/parse.y:6805
    #8 0x55a29cf36f25 in mrb_load_detect_file_cxt mrbgems/mruby-compiler/core/parse.y:6848
    #9 0x55a29cdba0a2 in main /mruby/mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:347
    #10 0x7f24ef43b0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #11 0x55a29cdb4a6d in _start (/mruby/bin/mruby+0x2a3a6d)

0x602000006998 is located 0 bytes to the right of 8-byte region [0x602000006990,0x602000006998)
allocated by thread T0 here:
    #0 0x7f24f01cfffe in __interceptor_realloc (/lib/x86_64-linux-gnu/libasan.so.5+0x10dffe)
    #1 0x55a29ceb9440 in mrb_default_allocf /mruby/src/state.c:68
    #2 0x55a29cdba747 in mrb_realloc_simple /mruby/src/gc.c:228
    #3 0x55a29cdba928 in mrb_realloc /mruby/src/gc.c:242
    #4 0x55a29cde12e5 in ht_init /mruby/src/hash.c:749
    #5 0x55a29cde2b8e in ht_rehash /mruby/src/hash.c:897
    #6 0x55a29cde379f in h_rehash /mruby/src/hash.c:996
    #7 0x55a29cde7f3d in mrb_hash_rehash /mruby/src/hash.c:1735
    #8 0x55a29ce77b62 in mrb_vm_exec /mruby/src/vm.c:1451
    #9 0x55a29ce5fa88 in mrb_vm_run /mruby/src/vm.c:981
    #10 0x55a29ceb87e1 in mrb_top_run /mruby/src/vm.c:2874
    #11 0x55a29cf36bdf in mrb_load_exec mrbgems/mruby-compiler/core/parse.y:6805
    #12 0x55a29cf36f25 in mrb_load_detect_file_cxt mrbgems/mruby-compiler/core/parse.y:6848
    #13 0x55a29cdba0a2 in main /mruby/mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:347
    #14 0x7f24ef43b0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
```

Ruby 2.5 is the oldest maintained version.

Explicit top level task to clarify in `tasks/test.rake`

Along with some refactoring on `mruby-math` tests.

When there is a corresponding tag, the `RBreak` object is used to make a global jump.

Like CRuby, it can't be caught by `rescue`.
It is also the same as CRuby that it can be canceled in the middle by `ensure`.

 ### How to find the corresponding tag with `throw`

The called `catch` method remains in the call stack, and the tag also remains in the stack at that time.
So it is possible to find the called location by searching the two.

Note that no method can be given to the `proc` object specified in `RBreak`.
Therefore, inside the `catch` method, the argument block is called in a seemingly meaningless closure.

Also, as a countermeasure against `alias` etc., the `proc` object, which is the body of the `catch` method, is saved when mrbgem is initialized.

Both MinGW and MSVC provide _WIN32 macro.

`compile_as_cxx` should generate a path under` conf.build_dir`

Ignore if `conf.enable_test` does not exist

No need to check class definition

Because if the configuration file didn't contain any `conf.enable_test`, `rake test` would report an exception and exit.

```console
% cat my_config.rb
MRuby::Build.new { toolchain }

% rake MRUBY_CONFIG=my_config.rb test

  ...SNIP...

rake aborted!
NoMethodError: undefined method `invoke' for nil:NilClass
/var/tmp/mruby/tasks/test.rake:24:in `block (3 levels) in <top (required)>'
Tasks: TOP => test => test:build => test:build:lib
(See full trace by running task with --trace)
```

Because now the `Struct` class is always defined when this file is included.

If the current directory is different from `MRUBY_ROOT` and it has` conf.build_dir` and `conf.enable_cxx_exception` set, it was generating a pathname outside of` build_dir`.
As a result, in some cases files unrelated to mruby could be linked.

```console
% pwd
/tmp/mruby/1/2/3/4/5/6

% mruby_dir=/tmp/mruby/a/b/c/d/mruby

% cat my_config.rb
MRuby::Build.new("host", "build/to/custom/directory") do
  toolchain
  enable_cxx_exception
end

% rake MRUBY_CONFIG=my_config.rb -f $mruby_dir/Rakefile > logs
% grep CXX logs
CXX   a/b/c/d/mruby/src/error-cxx.cxx -> a/b/c/d/mruby/src/error-cxx.o
CXX   a/b/c/d/mruby/src/gc-cxx.cxx -> a/b/c/d/mruby/src/gc-cxx.o
CXX   a/b/c/d/mruby/src/vm-cxx.cxx -> a/b/c/d/mruby/src/vm-cxx.o
CXX   a/b/c/d/mruby/mrbgems/mruby-compiler/core/codegen-cxx.cxx -> a/b/c/d/mruby/mrbgems/mruby-compiler/core/codegen-cxx.o
CXX   a/b/c/d/mruby/mrbgems/mruby-compiler/core/y.tab-cxx.cxx -> a/b/c/d/mruby/mrbgems/mruby-compiler/core/y.tab-cxx.o
CXX   ../a/b/c/d/mruby/src/error-cxx.cxx -> ../a/b/c/d/mruby/src/error-cxx.o
CXX   ../a/b/c/d/mruby/src/gc-cxx.cxx -> ../a/b/c/d/mruby/src/gc-cxx.o
CXX   ../a/b/c/d/mruby/src/vm-cxx.cxx -> ../a/b/c/d/mruby/src/vm-cxx.o
```

Exclude `<build-dir>/mrbc/**/*` from rake target rules

Make the argument of `Kernel#catch` optional as Ruby does

Prevents the auto-generated mrbc target source code from being compiled under host conditions.

This is because a build error occurred when `conf.enable_cxx_exception` was set.

Also implement the following changes.

* Add tests
* Use `Object#equal?` to compare tags for Ruby compatibility
* Use `attr_reader`

Fix message of `ThrowCatchJump` in `mruby-catch` gem

* Use `_Complex` instead of `complex` (MSYS2 do not support `complex`)
* Use `_Dcomplex` instead of `_Complex` on MSCV
* Avoid operator division and multiplication of complex

### Example

```ruby
begin
  throw 1
rescue Exception => e
  puts e.message
end
```

#### Before this patch:

```console
$ bin/mruby example.rb
uncaught throw :1
```
#### After this patch (same as Ruby):

```console
$ bin/mruby example.rb
uncaught throw 1
```

This gem uses C99 `_Complex` features. You need a C compiler that
supports `_Complex` to enable this gem. All `gcc`, `clang`, `VC` support
`_Complex` so there should not be a big problem.

* Use `class_eval` instead of `instance_eval`.
* Reduce `class_eval` calls
* define `Numeric#i`
* undefine `Complex#i`