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]

This reverts commit d5c88683.

Not only mruby, it is one of the difficult things to adjust the performance vs. memory efficiency of the software.
If the approximate target device is divided and the adjustment value for it is prepared by default, it is a good indicator to do fine adjustment.

This PR divides into four profiles.

***Caution: There is no basis for the definitions in the patch.***

- `MRB_CONSTRAINED_BASELINE_PROFILE` - for microprocessors. Reduce memory consumption.
- `MRB_BASELINE_PROFILE` - Default value of mruby.
- `MRB_MAIN_PROFILE` - For desktop computers. Assume that a huge amount of RAM, 10 MiB or more, is on board.
- `MRB_HIGH_PROFILE` - for servers. Assume that mruby VM has a long life.

As you can see the profile name has been ~~stolen~~ imitated from H.264.

Add/Edit ossfuzz config options

Move `Integral#(zero|nonzero|positive|negative)?` to `Numeric`

proto fuzzer: Add source files necessary to compile proto fuzzer

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

Fix `Numeric#to_r`

Add support for CC="gcc --option ..." again

Because these methods work if object is `Comparable`, and `Numeric` is
`Comparable`.

If $rake_root_fiber is used, sh runs command in another Fiber. If
command is ran in another Fiber, "rescue RuntimEerror" can't rescue
exception for system(...) failure.

How to reproduce:

    $ CC="gcc -std=gnu99" ./minirake
    (in /home/kou/work/ruby/mruby.kou)
    CC    mrbgems/mruby-compiler/core/codegen.c -> build/test/mrbgems/mruby-compiler/core/codegen.o
    sh: 1: gcc -std=gnu99: not found
    rake aborted!
    Command Failed: ["gcc -std=gnu99" -g -std=gnu99 ...]

`float op rational` should return `float`, since float is an inexact
value.