`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`

The former should contain function like methods, and the latter should
contain methods shared by all objects.

Reported by Sergey Ukrainskiy: https://github.com/mruby/mruby/commit/f5fb1307b017fb972c12b4ec4b1866d789b0ca09#r33590698

Because it is not currently output by `mrbc`.

Move `Object#(Rational|Complex)` to `Kernel`

Move `Numeric#__coerce_step_counter` to `Integral`

This method is only used in `Integral#step`

I assume there's no realistic usage of `Rational` with
`MRB_WITHOUT_FLOAT`. But just for consistency.

(Proof of Concept) mruby tuning profiles [ci skip]