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

The warnings were detected by cppcheck.

Integrate `mrb_str_inspect` and `mrb_str_dump`

Fix that `print` command raises `FrozenError` in `mrdb`; ref 1f5a7f2f

#### Before this patch:

  ```
  $ echo 'p true' | bin/mrdb /dev/null
  (/dev/null:1) mruby application exited.
  FrozenError: can't modify frozen String
  (-:0)
  ```

#### After this patch:

  ```
  $ echo 'p true' | bin/mrdb /dev/null
  (/dev/null:1) $1 = true
  (/dev/null:1)
  ```

More tests, more time.

Add mrbgem version field to lock file

Implement Ruby2.7's frozen strings from `Module#name`

Merge pull request #4760 from shuujii/use-mrb_str_concat-instead-of-mrb_str_to_str-plus-mrb_str_cat_str

Use `mrb_str_concat` instead of `mrb_str_to_str` + `mrb_str_cat_str`

Get keyword arguments with `mrb_get_args()`

Keyword arguments can now be retrieved with the `:` specifier and
`mrb_kwargs` data.

For the interface, I referred to CRuby's `rb_get_kwargs()`.
For implementation, I referred to `OP_KARG` or etc.

Refine the usage message of `mirb` command

Refine the usage message of `mruby` command