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`

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