As of CRuby2.7 it is only warned. `mruby` prohibits explicitly to
implement the future Ruby3 behavior.

The `debug` build target (`MRB_GC_STRESS` is enabled) on CI have been
compiled but not tested so far. However, I think testing with
`MRB_GC_STRESS` is effective (in fact, I found #4907 bug).

Therefore, I integrated `debug` and `full-debug` build targets to enable
`MRB_GC_STRESS` testing. Testing with `MRB_GC_STRESS` takes a little time,
but compiling takes more time, so CI execution time does not increase due to
decrease of build target.

Enable sanitizer in travis test

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`