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()`

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`.

`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`