`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`

Update ossfuzz options

Move `mrb_gc_arena_restore` to inside the loop in `mrb_file_s_chmod`

Merge pull request #4428 from shuujii/raise-TypeError-if-the-argument-type-is-unsupported-in-mrb_stat0

Raise `TypeError` if the argument type is unsupported in `mrb_stat0`

Use `mrb_string_value_cstr` in `mrb_str_to_dbl`

Refactor `mrb_str_to_cstr` and `mrb_string_value_cstr`

- Extract null byte check to function.
- Avoid string allocation if null byte is included.
- Use `str_new` instead of `mrb_str_dup` + `mrb_str_modify`

Avoid using `mrb_str_to_cstr` if possible

Because it always allocate new string. Replace with the followings:

- Use `RSRING_PTR` if string is guaranteed to be null-terminated.
- Use `mrb_string_value_cstr` or `mrb_get_args("z")` if return value isn't
  modified.

Use `mrb_fixnum_to_str()` instead of `Fixnum#to_s`

ossfuzz: Add simple mruby compile test harness

Merge pull request #4422 from shuujii/check-whether-object-is-immediate-in-mrb_gc_register-unregister

Check whether object is immediate in `mrb_gc_(register|unregister)`

Simplify conversion process for `i` in `mrb_get_args()`

Fix `FLAG_SRC_STATIC` always set in `mrb_read_irep()` with `MRB_USE_CUSTOM_RO_DATA_P`

Clean duplicate code

Removed duplicates in the code entered for "Concatenate string literal".

Merge pull request #4417 from shuujii/unify-overflow-error-class-for-conversion-to-integer-to-RangeError

Unify overflow error class for conversion to integer to `RangeError`

Small fix in `mruby-bin-mruby`

Remove unneeded `argc` check in `mrb_str_aref_m()`