Introduce `mrb_ssize` type for buffer size on memory; ref #4483

Previously, `mrb_int` was used as the type that represents the buffer size
on memory, but the sizes of `RString` and `RArray` exceed 6 words when
`MRB_INT64` is enabled on 32-bit CPU.

I don't think it is necessary to be able to represent the buffer size on
memory that exceeds the virtual address space. Therefore, for this purpose,
introduce `mrb_ssize` which doesn't exceed the sizes of `mrb_int` and
pointer.

I think all `mrb_int` used for this purpose should be changed to
`mrb_ssize`, but currently only the members of the structures (`RString`,
`mrb_shared_string`, `RArray` and `mrb_shared_array`) are changed.

Always enable the rational and complex literals

Release `mruby 2.1.0`.

I think they can always be enabled because the regular expression literal is
always enabled.

Refactor `mrb_string_value_cstr`

- Keep `MRB_STR_ASCII` flag.
- Avoid a string object creation.

Fix argument specs to `Kernel`

Revert "Implement Ruby2.7's frozen strings from `Symbol#to_s`"

This feature was reverted from Ruby 2.7.

Fix here document with EOF

#### Before this patch:

  ```
  $ bin/mruby -e 'p <<EOS
1
EOS'
  -e:4:0: can't find heredoc delimiter "EOS" anywhere before EOF
  -e:4:0: syntax error, unexpected $end, expecting tHEREDOC_END or tHD_STRING_PART or tHD_STRING_MID
  ```

#### After this patch (same as Ruby):

  ```
  $ bin/mruby -e 'p <<EOS
1
EOS'
  "1\n"
  ```

Use `mrb_intern_lit` if possible in `parse.y`

Use `intern` instead of `intern_cstr` if possible in `parse.y`

Fix argument specs to `Integer`

Defer getting `caller_dir` until needed in `MRuby::LoadGems#gem`

Fix argument specs to `IO`

Remove unneeded dependency from `mruby-hash-ext`

This is caused by incomplete fix in #4746

On cases like `a.unshift(*a)`.

Clear `MRB_STR_SHARED` flag in `mrb_str_modify_keep_ascii`; close #4807

Drop test dependency from `mruby-hash-ext` to `mruby-enumerator`

Merge pull request #4805 from shuujii/remove-test-that-depend-on-mruby-enumerator-from-mruby-array-ext

Remove test that depend on `mruby-enumerator` from `mruby-array-ext`

`Object.const_defined?(:Enumerator)` is always false because
`mruby-enumerator` is not specified in `test_dependency`. I don't
think this test is necessary.

Avoid using C++ style comments (`//`) in `parse.y` [ci skip]