Set `MRB_STR_ASCII` flag to some stringize methods

- It can now deal with operands in the range of `OP_EXT*`.
- It can now call the same method as the variable name without arguments.

  ```ruby
  def a
    "Safe!"
  end

  a = "Auto!"

  eval "a()" # call method `a`
  ```

Get local variable names from orphan block; ref #3710

Fix float constant redefinition warnings

Merge pull request #5011 from shuujii/respect-CFLAGS-CXXFLAGS-and-LDFLAGS-env-vars-in-appveyor_config.rb

Respect CFLAGS, CXXFLAGS, and LDFLAGS env vars in appveyor_config.rb

[skip travis]

We made it `64` which should be big enough:
- Format modifier: 4 characters max
- Maximum width: 19 digits max
- Period between width and precision: 1 character
- Maximum precision: 19 digits max
- Format specifier: 1 character
- NUL terminator: 1 byte
- Total: 45 < 64

Detect newly added mrbgems when re-creating gem_init.c

Merge pull request #5009 from shuujii/removed-Bison-related-settings-from-.travis.yml-and-appveyor.yml

Removed Bison related settings from `.travis.yml` and `appveyor.yml`

Revert part of "Start GitHub Actions" (4ce3997c)

Because some changes have been overridden.

Add `y.tab.c` to remove Bison from build dependencies; ref 4ce3997c

I sometimes see Bison related problems in setting up build environments.
Therefore to remove Bison from build time dependencies, add `y.tab.c`
generated by Bison to the repository.

The reduction of dependency at build time also reduces the labor and time
for setup and installation in CI.

In addition, a path in `#line` directive is converted to a relative path so
that its path is constant regardless of development environments.

`Windows-MinGW` and `Windows-VC` also requires updates.

Do not destruct rest arguments for __send__

Formerly, `__send__(*args)` modified `args` with `Array#shift`.
This bug affects optcarrot.

This changeset avoids the array destruction by using
`args = args[1, len-1]`.

Retry `rake -m` on AppVeyor [skip travis]

The following error occurs when using `rake -m` on AppVeyor:

  ```
  fatal error C1041: cannot open program database 'C:\projects\mruby\vc140.pdb'; if multiple CL.EXE write to the same .PDB file, please use /FS
  ```

Therefore, the issue is solved by not creating the PDB file. It is expected
to be somewhat faster by not generating debugging information (I don't think
debugging information is necessary for normal CI).

This reverts commit ac68c908.
`rake -m` does not work for VisualC.

Adding warnings for mrb_load functions leaking RProc objects

Unify `eql?` receiver in `Hash` according to Ruby

### Example

  ```ruby
  # example.rb
  class A
    def eql?(o)
      p self.class
      super
    end
    def hash
      1
    end
  end

  class B < A; end

  h = {A.new => 1}
  h[B.new]
  ```

#### Before this patch:

  ```console
  $ bin/mruby example.rb
  A
  ```

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

  ```console
  $ bin/mruby example.rb
  B
  ```

Instead we added `%define parse.error verbose`.

The newer `clang` warns implicit float conversions.

Get an irep symbol if it's `OP_GETSV` or `OP_SETSV`