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

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.

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

Fix typo in `mrblib/range.rb` [ci skip]

Need explicit cast for C++

Fix missing assertions in `mruby-objectspace` test

Comment out the empty `Array#bsearch_index` test

Fix missing assertions in `mruby-fiber` test

`Kernel#itself` should not be a module function

This time, the allocated memory comes from the string object, which is
referenced from GC arena. The memory region will be reclaimed when the C
function called from VM is terminated, or the GC arena is restored.