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.

Use `mrb_str_cat_str` instead of `mrb_str_concat` if possible

Fix that `Module#to_s` may return frozen string; ref 08eafe21

Print mrbc command on verbose mode

Merge pull request #4778 from shuujii/move-methods-of-Kernel-to-kernel.rb-from-io.rb-in-mruby-io-gem

Move methods of `Kernel` to `kernel.rb` from `io.rb` in `mruby-io` gem

Remove constant export declaration for MRBC output compiled as C

Make `IO#each` family without block to return `Enumerator`

Merge pull request #4775 from shuujii/use-mrb_sym_name_len-instead-of-mrb_sym_name-in-assign_class_name

Use `mrb_sym_name_len` instead of `mrb_sym_name` in `assign_class_name`

Adjust `buf` size in `str_escape`

Fix the example of `Array#intersection` in the document [ci skip]