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`.

Add tests for #4746

Fix opcode semantics comment miss.

For example, local variables in the following def:
```ruby
def foo(a = (not_set = true), &block)
  ...
end
```
should be `a, block, not_set`, but were `a, not_set, block`.

Avoid `symhash()` call for inline symbol in `sym_intern()`

Escape the AST string

Dump more node types in `mrb_parser_dump`

#### Before this patch:

  ```terminal
  $ bin/mruby -v -e '%w[1 2]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_WORDS:
  00001       NODE_STR "1" len 1
  00001       node type: 85 (0x55)
  00001       NODE_STR "2" len 1
  (snip)
  ```

#### After this patch:

  ```terminal
  $ bin/mruby -v -e '%w[1 2]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_WORDS:
  00001       NODE_STR "1" len 1
  00001       NODE_LITERAL_DELIM
  00001       NODE_STR "2" len 1
  (snip)
  ```

#### Before this patch:

  ```terminal
  $ bin/mruby -v -e '%i[1]; %I[#{2}]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     node type: 87 (0x57)
  00001     node type: 87 (0x57)
  (snip)
  ```

#### After this patch:

  ```terminal
  $ bin/mruby -v -e '%i[1]; %I[#{2}]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_SYMBOLS:
  00001       NODE_STR "1" len 1
  00001     NODE_SYMBOLS:
  00001       NODE_STR "" len 0
  00001       NODE_BEGIN:
  00001         NODE_INT 2 base 10
  00001       NODE_STR "" len 0
  (snip)
  ```

#### Before this patch:

  ```terminal
  $ bin/mruby -v -e '%w[1]; %W[#{2}]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     node type: 86 (0x56)
  00001     node type: 86 (0x56)
  (snip)
  ```

#### After this patch:

  ```terminal
  $ bin/mruby -v -e '%w[1]; %W[#{2}]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_WORDS:
  00001       NODE_STR "1" len 1
  00001     NODE_WORDS:
  00001       NODE_STR "" len 0
  00001       NODE_BEGIN:
  00001         NODE_INT 2 base 10
  00001       NODE_STR "" len 0
  (snip)
  ```

Remove a trailing space in `MRUBY_DESCRIPTION`

#### Before this patch:

  ```terminal
  $ bin/mruby -v -e ':"#{1}"; ["#{2}": 0]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     node type: 83 (0x53)
  00001     NODE_ARRAY:
  00001       NODE_KW_HASH:
  00001         key:
  00001           node type: 83 (0x53)
  00001         value:
  00001           NODE_INT 0 base 10
  (snip)
  ```

#### After this patch:

  ```terminal
  $ bin/mruby -v -e ':"#{1}"; ["#{2}": 0]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_DSYM:
  00001       NODE_DSTR:
  00001         NODE_STR "" len 0
  00001         NODE_BEGIN:
  00001           NODE_INT 1 base 10
  00001         NODE_STR "" len 0
  00001     NODE_ARRAY:
  00001       NODE_KW_HASH:
  00001         key:
  00001           NODE_DSYM:
  00001             NODE_DSTR:
  00001               NODE_STR "" len 0
  00001               NODE_BEGIN:
  00001                 NODE_INT 2 base 10
  00001               NODE_STR "" len 0
  00001         value:
  00001           NODE_INT 0 base 10
  (snip)
  ```

Remove unnecessary type `mrb_hash_value`