Defer getting `caller_dir` until needed in `MRuby::LoadGems#gem`

Fix argument specs to `IO`

Remove unneeded dependency from `mruby-hash-ext`

This is caused by incomplete fix in #4746

On cases like `a.unshift(*a)`.

Clear `MRB_STR_SHARED` flag in `mrb_str_modify_keep_ascii`; close #4807

Drop test dependency from `mruby-hash-ext` to `mruby-enumerator`

Merge pull request #4805 from shuujii/remove-test-that-depend-on-mruby-enumerator-from-mruby-array-ext

Remove test that depend on `mruby-enumerator` from `mruby-array-ext`

`Object.const_defined?(:Enumerator)` is always false because
`mruby-enumerator` is not specified in `test_dependency`. I don't
think this test is necessary.

Avoid using C++ style comments (`//`) in `parse.y` [ci skip]

Remove unused enum in `mrb_cstr_to_dbl`

Fix argument specs to `Hash`

Fix argument specs to `File`

Fix argument specs to `Fiber`

Fix `git` command parameter for Windows.

Remove unneeded `Array` creation in `Struct#_inspect`

Remove documents about `Regexp` argument from `String#{[],[]=}` [ci skip]

Refine `String#split` document

Merge pull request #4791 from shuujii/optimize-chars2bytes-with-MRB_UTF8_STRING-to-ASCII-only-string

Optimize `chars2bytes` with `MRB_UTF8_STRING` to ASCII only string

### Benchmark (with `MRB_UTF8_STRING`)

  ```
  $ mruby -e '
    COUNT = 150000
    SIZE = 10000
    strs = Array.new(COUNT) do
      s = "a" * SIZE
      s.size  # set `MRB_STR_ASCII` flag
      s
    end
    i = 0
    t = Time.now
    while i < COUNT
      strs[i][-2..-1] = ""
      i += 1
    end
    printf "%.2f sec\n", Time.now - t
  '

  1.10 sec  # before
  0.07 sec  # after
  ```

Squeeze redundant spaces in `test/t/numeric.rb` [ci skip]