Reverses UTF-8 strings without allocated heap for working memory.

 1. String before reversing:
    ```
    "!yburmの界世"
    # byte unit
    [33, 121, 98, 117, 114, 109, 227, 129, 174, 231, 149, 140, 228, 184, 150]
    ```
 2. Reverse the byte order of each character:
    ```
    [33, 121, 98, 117, 114, 109, 174, 129, 227, 140, 149, 231, 150, 184, 228]
    ```
 3. Reverse the whole byte order and complete:
    ```
    [228, 184, 150, 231, 149, 140, 227, 129, 174, 109, 114, 117, 98, 121, 33]
    # string
    "世界のmruby!"
    ```

Add ISO section number to `Kernel.#local_variables` [ci skip]

Move `Kernel#__send__` test to core from `mruby-metaprog`

Remove unneeded `mrb_str_dup()` in `Module#name`

Fix path of `error.h`.

`mrb_class_path()` always returns a new string or `nil`.

Merge pull request #4508 from shuujii/fix-cvar-ivar-const-and-method-can-be-removed-to-frozen-object

Fix cvar, ivar, const and method can be removed to frozen object

Fix index in error message of `Struct#[]`

Adjust allocation size in `mrb_id_attrset()` (`mruby-struct`)

Before this patch:

  $ bin/mruby -e 'Struct.new(:a,:b).new[-3]'
  #=> offset -1 too small for struct(size:2) (IndexError)

After this patch (same as Ruby):

  $ bin/mruby -e 'Struct.new(:a,:b).new[-3]'
  #=> offset -3 too small for struct(size:2) (IndexError)

Remove a meaningless branch condition in `mruby-struct`

The following branch condition is always true:

  // mrbgems/mruby-struct/src/struct.c:187 in make_struct_define_accessors()
  if (is_local_id(mrb, name) || is_const_id(mrb, name)) {

Fix class name validation in `Struct.new`

Before this patch:

  $ bin/mruby -e 'p Struct.new("A-")'
  #=> Struct::"A-"

After this patch:

  $ bin/mruby -e 'p Struct.new("A-")'
  #=> NameError: identifier A- needs to be constant

`Kernel#global_variables` should not include undefined `$1`-`$9`

Fix typo in `String#setbyte` error message

Remove redundant colon in `Proc#inspect` (`mruby-proc-ext`)

Before this patch:

  $ bin/mruby -e 'p proc{}'  #=> #<Proc:0x7fd5eb8206d0@-e::1>

After this patch:

  $ bin/mruby -e 'p proc{}'  #=> #<Proc:0x7fd5eb8206d0@-e:1>

Replace obsolete macros

- They are not include in Ruby.
- Appear in duplicate when `$1`-`$9` are defined.

Remove `Kernel#global_variables` from core

Clarify `mruby-(kernel|object)-ext` gem summary; ref 8e637bdd [ci skip]

Fix missing assertions in `test/t/syntax.rb`

Remove "Check the usage of a NUL character" test

Because there is not assertion in this test and NUL character literal is
used in other tests.

This method is defined in `mruby-metaprog` gem.