Replace obsolete macros

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.

Fix `Rational#==(Complex)`

Fix missing assertions in `mruby-pack` test

Merge pull request #4474 from shuujii/use-undefined.equal-obj2-instead-of-obj2-eq-undefined-in-assert.rb

Use `$undefined.equal?(obj2)` instead of `obj2 == $undefined` in `ass…

Merge pull request #4473 from shuujii/make-some-functions-static-in-mrbgems-mruby-metaprog-src-metaprog.c

Make some functions static in `mrbgems/mruby-metaprog/src/metaprog.c`

In case `obj2.==` is broken.

Consider a Numreic class like `Complex` that does not have `<=>` but `==`
works (`0i == 0r` is `true`).

Remove `mrb_alloca()` function

Move `mrb_mod_s_nesting()` to `mruby-metaprog` gem from the core

Add `Complex.rect`