In case `obj2.==` is broken.

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

Or `Float` if `mrb_float` value is too big (or too small) to fit in
`mrb_int`. The `_int_` in `mrb_int_value` means `Integral` module, which
represents integer-like values in mruby.

It should raise an error.

This commit removes `Float` from `rational.c`.

Move `**`,`/`,`quo`,`div` and comparison methods to Integral from Numeric

Having these methods in Numeric can get in the way of creating subclasses of
Numeric because they only support Fixnum and Float.

Revert part of #4457

Merge pull request #4457 from shuujii/change-the-order-of-expected-and-actual-in-mruby-rational-test

Change the order of "expected" and "actual" in `mruby-rational` test

Drop dependency from `mruby-rational` to `mruby-object-ext`

New functions:
* mrb_num_plus(mrb, x, y)
* mrb_num_minus(mrb, x, y)
* num_num_mul(mrb, x, y)

Remove "LINE" section reader

Fix `Rational#<=>(Numeric)`

Move `Kernel#instance_eval` to `BasicObject`

Move `Kernel#equal? to `BasicObject`

Move `Kernel#instance_exec` to `BasicObject`