Merge mruby 2.1.1

- `compile_flags.txt'
- `compile_commands.json`

Align RIStruct data for rational/complex

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]`.