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]

Refactor `mrb_class_name_class`

- Use `mrb_sym_name_len` instead of `mrb_sym_name` (class name should not
  be escaped).
- Avoid `mrb_str_dup` (it is unnecessary to be shared string because it is
  changed).

SHARED string is not required when sharing POOL string

Rename `str_make_shared()` to `str_share()` in `src/string.c`

The heap string buffer of POOL string always exists, does not need to be
released, and read only, so it can be shared as NOFREE string.

Because it may not create `struct mrb_shared_string`.

Besides that fix bugs that mistakenly calls `raise_error` that emits
code to raise runtime error instead of `codegen_error` that terminates
code generation immediately.

Remove unused exception classes in `mruby-io` gem