Ignore `mruby-bin-config` in cross-building

🔒 Fix missing HTTPS on links

🔒 Fix missing HTTPS on links

This is because `MRuby::Build.currrent` and `MRuby::CrossBuild.current` are assignments to different instance variables.

feat(CI): add a GitHub Action to check spelling

- Fix spelling
- Run only on pull request
- Using https://github.com/client9/misspell

feat(CI): add a GitHub Action to lint the YAML

Rename build configuration files for CI

Currently, there are build configuration files for CI, `travis.rb` and
`appveyor.rb`, but they are used for GCC/Clang and MSVC, not for Travis CI
and Appveyor, respectively.

Therefore, rename them to `gcc-clang.rb` and `msvc.rb`, respectively, and
move them under `build_config/ci/` to clarify that they are for CI.

Guess toolchain when `MRuby::Build#toolchain` argument is omitted

- Run only on pull request
- Add a `.yamllint` config file
- Lint YAML

Ensure initialization of `RVALUE_zero` in `mrb_obj_alloc`

Output build log at the time of installation

refactor: remove trailing whitespace from C, Header, Ruby and YAML files

Lint

Union initialization initializes the first member. The first member of
`RVALUE` is `struct free_obj`, but because it is only 4-words, it seems that
initialization after the 5th word is not ensured.

Therefore, I created 6-words `struct RVALUE_initializer` for initialization
and made it the first member.

Fix spelling

```ruby
def method_missing(meth, ...)
  send(:"do_#{meth}", ...)
end
```

They used to be `size_t`, `uint32_t` and `ptrdiff_t`. Now all of them
made to be `uint32_t`.

Merge pull request #5217 from shuujii/call-super-before-creating-host-build-in-MRubyCrossBuild-initialize

Call `super` before creating `host` build in `MRuby::CrossBuild#initialize`

Call `super` (`block`) ahead so that creation of the `host` build can be
skipped if pre-built `mrbc` can be specified in the future.

close #5213.

Allow obsoluted `MRB_METHOD_T_STRUCT`

Remove unused `@endian` in `MRuby::CrossBuild#initialize`

Update "Configuration Options Changed" in `doc/mruby3.md` [ci skip]

Make it possible that `libmruby.a` is not created

Previously, `libmruby.a` was created even if only `mruby-bin-mrbc` or`
mruby-compiler` was specified for gem, but by specifying `disable_libmruby`,
the creation of `libmruby.a` can be suppressed.

### Note

The https://github.com/mruby/mruby/pull/5084#issuecomment-723521971
incompatibility seems to be difficult for users to avoid, so the original
behavior has been restored. Therefore, if we need `mrbc` other than the
`host` build, we need to explicitly specify `mruby-bin-mrbc` gem.

Due to the above changes, `build_config/boxing.rb` etc. will not work, but I
have added` mruby-bin-mrbc` to `default.gembox` to fix it. I don't think
this change is a big deal because originally `mruby-compiler` was included.

Adjust insertion point in `fix_prepend_module()`.

Module#include and Module#prepend now affect classes and modules
that have already included or prepended the receiver, mirroring the
behavior if the arguments were included in the receiver before
the other modules and classes included or prepended the receiver.

```ruby
class C; end
module M1; end
module M2; end
C.include M1
M1.include M2
p C.ancestors #=> [C, M1, M2, Object, Kernel, BasicObject]
```