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

Merge pull request #4752 from shuujii/drop-initialization-dependency-from-mruby-print-to-mruby-sprintf

Drop initialization dependency from `mruby-print` to `mruby-sprintf`

In the old implementation, `Kernel#printf` raise error if `mruby-sprintf`
gem isn't specified before `mruby-print` gem. The new implementation
eliminates this ordering issue. This way is the same as `Kernel#printf` and
`IO#printf` in `mruby-io` gem.

This is an experimental changes in Ruby 2.7.

Remove unused code to old Visual Studio in `tasks/toolchains/visualcpp.rake`

Remove duplicates header files in `src/pool.c`

These are included in `mruby.h`.

As a background, if `enable_cxx_abi` is specified, the macro that
defines the maximum value in `stdint.h` is undefined depending on the
environment.

- Confirmed with gcc on FreeBSD 12.0 and mingw32-gcc available on
  FreeBSD.
- `INTPTR_MAX`, `INT64_MAX` and `UINT64_MAX` are defined by `cstdint`
  added in C++11. But `toolchains :gcc` adds `-std=c++03`, so the macros
  are not defined.
- To have these defined in `C++03`, `__STDC_CONSTANT_MACROS` must be
  defined in advance. This is already done by `mruby.h`.