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]

Fix argument specs to `Exception`

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

### Benchmark (with `MRB_UTF8_STRING`)

  ```ruby
  # benchmark.rb
  COUNT = 300000
  SIZE = 10000
  s = "a" * SIZE
  s.size  # set `MRB_STR_ASCII` flag
  i = 0
  while i < COUNT
    s[-1]
    i += 1
  end
  ```

#### Before this patch:

  ```
  $ time mruby benchmark.rb
          2.06 real         2.05 user         0.00 sys
  ```

#### After this patch:

  ```
  $ time mruby benchmark.rb
          0.05 real         0.04 user         0.00 sys
  ```

Fix incorrect `MRB_STR_ASCII` flag update in `mrb_str_dump`

### Example (with `MRB_UTF8_STRING`)

  ```ruby
  s = "\u3042"
  p s.size
  s.dump
  p s.size
  ```

#### Before this patch:

  ```
  1
  3
  ```

#### After this patch:

  ```
  1
  1
  ```

Remove `Kernel#getc`

`Kernel#getc` has been removed since Ruby 1.9 and is not defined in ISO.