`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)`

For the following reasons:

- Ruby compatibility.
- Add UTC offset (time zone informations was not included by #4433).
- More readable.

Example:

  Before this patch:

    p Time.gm(2003,4,5,6,7,8,9)        #=> Sat Apr  5 06:07:08 2003
    p Time.local(2013,10,28,16,27,48)  #=> Mon Oct 28 16:27:48 2013

  After this patch:

    p Time.gm(2003,4,5,6,7,8,9)        #=> 2003-04-05 06:07:08 UTC
    p Time.local(2013,10,28,16,27,48)  #=> 2013-10-28 16:27:48 +0900

Implementation:

I use `strftime(3)` because UTC offset can be added and program size become
smaller than the other implementations (using `sprintf(3)`, self conversion
etc) in my environment.

Fix `Time#(asctime|ctime)` according to ISO Ruby

- A leading charactor for day is space.
- Time zone does not included.

Before this patch:

  Time.gm(1982,3,4,5,6,7).asctime  #=> "Thu Mar 04 05:06:07 UTC 1982"

After this patch:

  Time.gm(1982,3,4,5,6,7).asctime  #=> "Thu Mar  4 05:06:07 1982"

Fix missing assertions in `mruby-time` test

Use `mrb_ensure_string_type` in `mrb_to_str`