Fix potentially use of wrong method cache

#### Example (with `MRB_METHOD_CACHE`)

  ```ruby
  GC.start
  c = Class.new
  p c            #=> #<Class:0x7fd6a180e790>
  c.new          #=> cache `c.new`
  c = nil
  GC.start       #=> `c` is GCed
  r = Range.dup
  p r            #=> #<Class:0x7fd6a180e790>
                 #   [same pointer as `c`]
  r.new(2, 3)    #=> ArgumentError: 'initialize':
                 #   wrong number of arguments (2 for 0)
                 #   [`c.new` is called instead of `r.new`]
  ```

#### Cause

  An entry of method cache is identified by class pointer and method
  id. However, reusing memory after GC may create a class with the same
  pointer as the cached class.

#### Treatment

  Cleared method caches of the class when the class is GCed.

Refine the assertion failure message in mrdb print tests

#### Before this patch:

  ```console
  Fail: mruby-bin-debugger(print) error (mrbgems: mruby-bin-debugger)
   - Assertion[2]
      Expected true to be false.
  ```

#### After this patch:

  ```console
  Fail: mruby-bin-debugger(print) error (mrbgems: mruby-bin-debugger)
   - Assertion[2]
      Expected "$2 = undefined method 'bar' (NoMethodError)\n" to be start_with? "$2 = (eval):2: undefined method".
  ```

Add double quotes for cygwin filenames #4904

Use GNU extension in C++ for cygwin

Handle CR LF newline natively in lexer

Parallelize compilation only on Travis CI

Parallel execution of tests makes log difficult to see due to mixing.

Use `git checkout` instead of `git reset`

With this change, if the checkout fails, it will stop with an error.

The purpose is to avoid deleting working branch history when
developing gem.

Refine `.travis.yml`

Fix "undefined method `Pathname'"; fix #4895

Use `exec` instead of `system` in `minirake` for exit status

* Use `rake` instead of `minirake`.
* Remove `gperf` configuration.

Execution time seems to be reduced by about 15%.

#### Before this patch:

  ```console
  $ ./minirake --foo; echo $?
  invalid option: --foo
  0
  ```

#### After this patch:

  ```console
  $ ./minirake --foo; echo $?
  invalid option: --foo
  1
  ```

Perform `shellquote` on referenced string

`mruby` does not warn like `CRuby` for cases like #4893.
Fix #4890, fix #4891, fix #4893.

`_0` is not numbered parameter

#### Before this patch:

  ```console
  $ bin/mruby rb -e '_0=:l; p ->{_0}.()'
  -e:1:13: _0 is not available
  -e:1:13: syntax error, unexpected $end, expecting '}'
  ```

#### After this patch (same as Ruby):

  ```console
  $ bin/mruby rb -e '_0=:l; p ->{_0}.()'
  :l
  ```

Fix potentially crash in `%n` of `mrb_vformat()` with 64-bit `int`

If `mrb_sym` is smaller than `int`, it is promoted to `int`.

Avoid method in method

Remove unnessesary branches

Fix for `#methods` to include methods that were `undef`

Fix `mruby-bin-debugger` tests; ref d2f2f9db

Simplify `print_backtrace()`

Avoid creating `Data` object that refers `mruby` objects.
Also close #4622 ref #4613

This reverts commit f507ff48.
It makes AppVeyor tests fail.