A part of a fix for issue mruby/mruby.github.io#50

Silence unused label warnings from gcc; ref #4524

  mruby/mruby/src/string.c:1722:4: warning: label 'bytes' defined but not used [-Wunused-label]
      bytes:
      ^~~~~

Change to UTF-8 string reversing with in place

Fix `String#byteslice` with `MRB_UTF8_STRING` and some edge cases

Fix rational new on 32 bits mode

Fix complex new on 32 bits mode

Example:

  $ bin/mruby -e '
    p "あa".byteslice(1)
    p "bar".byteslice(3)
    p "bar".byteslice(4..0)
  '

  Before this patch:

    "a"
    ""
    RangeError (4..0 out of range)

  After this patch (same as Ruby):

    "\x81"
    nil
    nil

Fix `struct RRange` overflow on 32-bit CPU with `MRB_NAN_BOXING`

Remove unneeded statement in `SET_OBJ_VALUE` with `boxing_word.h`

Fix argument specs to `Array`

Merge pull request #4530 from shuujii/compare-obj-pointer-directly-instead-of-using-mrb_obj_eq-in-mrb_gc_unregister

Compare obj pointer directly instead of using mrb_obj_eq in mrb_gc_unregister

The #4520 tried to address the issue, but it changes the type of
`mrb_check_frozen` argument; close #4520

Because immediate values are not registered.

Refine `Hash#rehash` example [ci skip]

Use stack memory for small name of Struct members

Fix potential overflow in `utf8len()`

Now to be calls `mrb_str_modify()` only once when 2 or more characters.

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!"
    ```

Previous example doesn't work because string key (frozen) can't be modified.

For example on 32 bit mode, when `p = 0xfffffffd`, `e = 0xfffffffe`
and `len = 4`, the sum of `p` and `len` can be to `1`, and comparison
with `e` will to be false.

As a result, a segmentation fault occurs by referring to address 0.

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

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

`(r).value.bp` and `v` have the same value due to assignment of the line
preceding the removed line.

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

Fix path of `error.h`.