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`

Fix double rounded by negative index for `String#[]`

When I found this function, I expected it to behave the same as the
`alloca(3)` function, but it is accually the `mrb_alloca()` function
does not free the heap until the `mrb_close()` function is called.

Also, even if it is deleted, it can be replaced with the combination
of the `MRB_TT_DATA` object and the `mrb_gv_set()` function if it is
sure necessary.

- Before patched:
  ```
  $ mruby -e 'p (-12..-1).map { |i| "Hello"[i] }.join'
  "HelloHello"
  ```
- After patched:
  ```
  $ mruby -e 'p (-12..-1).map { |i| "Hello"[i] }.join'
  "Hello"
  ```

Name the return value of `mrb_range_beg_len()`

Fix the order of "expected" and "actual" in `mruby-time` test

Freeze `Rational` and `Complex` objects

`Kernel#Rational` requires numerator

Read irep from buffers