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

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`

Check if `.pi` build rules are defined

`.pi` files are created for `.o` files that `build.products` depends on, but
an error will occur if the build rule is unknown, so add a check.

I don't think this situation would normally arise. However, in
`mattn/mruby-onig-regexp`, when using bundled onigmo, onigmo's `.o` files
are added to dependency of `libmruby.a` in the second and subsequent builds,
and mruby does not know the build rule, so the following error had occured.

```console
rake aborted!
Don't know how to build task '/mruby/build/host/mrbgems/mruby-onig-regexp/onigmo-6.2.0/libonig_objs/ascii.pi' (See the list of available tasks with `rake --tasks`)
```

Fix breakage where builds except `host` don't refer` mrbc` in `host`

Build error occurred under the following conditions.

* `host` build is presym enabled and unspecified `mruby-bin-mrbc` gem.
* Build except `host` unspecified `mruby-bin-mrbc` gem and mrbcfile.

### Example

#### Build configuration

```ruby
MRuby::Build.new{|conf| conf.toolchain}
MRuby::Build.new("host2"){|conf| conf.toolchain}
```

#### Error

```console
rake aborted!
external mrbc or mruby-bin-mrbc gem in current('host2') or 'host' build is required
/mruby/lib/mruby/build.rb:326:in `mrbcfile'
/mruby/tasks/mrblib.rake:9:in `block in <top (required)>'
/mruby/lib/mruby/build.rb:18:in `instance_eval'
/mruby/lib/mruby/build.rb:18:in `block in each_target'
/mruby/lib/mruby/build.rb:17:in `each'
/mruby/lib/mruby/build.rb:17:in `each_target'
/mruby/tasks/mrblib.rake:1:in `<top (required)>'
/mruby/Rakefile:27:in `load'
/mruby/Rakefile:27:in `<top (required)>'
```

We don't want to increase number of files that should be compiled by C++
compiler when `enable_cxx_exception` is turned on.

We have introduced following new instructions.

 * `OP_LAMBDA16`
 * `OP_BLOCK16`
 * `OP_METHOD16`
 * `OP_EXEC16`

Each instruction uses 16 bits operand for `reps` index. Since new
instructions are added, `mruby/c` VM should be updated.

Due to new instructions, dump format compatibility is lost, we have
increased `RITE_BINARY_MAJOR_VER`.

In addition, we have decreased the size of `refcnt` in `mrb_irep` from
`uint32_t` to `uint16_t`, which is reasonably big enough.

In addition, stop eager allocation of `mt` table.

Introduced `MRB_PRESYM_INIT_SYMBOLS()`

Use `mrb_open_core()` instead of `mrb_open()`  in `mrbc`

Remove unnecessary `ci0` variables; ref #5272

Remove unnecessary configuration macros; ref #5060

The `init_SYMBOLS()` function implicitly defined in `MRB_PRESYM_DEFINE_VAR_AND_INITER()` requires some familiarity when trying to find it from the caller.
By introducing `MRB_PRESYM_INIT_SYMBOLS()`, it is possible to find directly from the identifier.

The following macros will be removed:
- `ENSURE_STACK_INIT_SIZE`
- `RESCUE_STACK_INIT_SIZE`
- `MRB_ECALL_DEPTH_MAX`

Change `.o.d` back to `.d` because `.pi.d` is no longer created

Build internal mrbc in an internal directory

Fixed `String#unpack` to handle the highest range of integer values

This is a correction based on the review by @matz.
https://github.com/mruby/mruby/pull/5306#pullrequestreview-578378401