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

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.

* mrb_fixnum_plus()
* mrb_fixnum_minus()
* mrb_fixnum_mul()

This function returns `Fixnum` if the value fits in `mrb_int`,
otherwise it returns `Float` value (mruby behavior of handling integers).

Fix `Rational#==`

It used heap allocated memory for the parser stack, but there's
possibility of parser termination by exceptions. In that case, the
parser stack memory is leaked.

We were afraid of stack consumption, but parser stack size is only 4KB,
so we don't have to worry about it (at least for the parser).

The previous value (`UINT16_MAX`) was too long for symbols, so it raises
an exception after the length check.

Fix typo in `mrbgems/mruby-io/src/file_test.c` [ci skip]

Fix name assignment to frozen anonymous class/module

`String#each_byte` is not defined in ISO Ruby but it is implemented in
the core mruby because it's useful.

Refine `Time#(to_s|inspect)`