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()`

- Modify some error messages for consistency.
- Add test for codegen error.
- Use regular expression for error message matching in test.

Refine error message output for `mruby` command

- Write message to stderr instead of stdout.
- Avoid duplicate message output (`SyntaxError`, `ScriptError` etc).
- Refine invalid option message.
- Suppress redundant usage output.
- Fix some incorrect exit code.

Fix missing assertions in `mruby-io` test

Refine the default values of `flunk`

The default message for the second argument should be set to the first
argument because only one argument is normally specified.

Commented out "Struct.new removes existing constant" test

Because this test is always skipped.

Update document for `MRB_USE_CUSTOM_RO_DATA_P` and some configurations

- (Modify) `MRB_INT16`
- (Add)    `MRB_INT32`
- (Modify) `MRB_INT64`
- (Add)    `MRB_USE_ETEXT_EDATA`
- (Add)    `MRB_NO_INIT_ARRAY_START
- (Add)    `MRB_WITHOUT_FLOAT`
- (Add)    `MRB_METHOD_CACHE`
- (Add)    `MRB_METHOD_CACHE_SIZE`
- (Add)    `MRB_METHOD_TABLE_INLINE
- (Add)    `MRB_ENABLE_ALL_SYMBOLS`

Remove duplicated `String#each_char`

Add customized `mrb_ro_data_p()`

User definable `mrb_ro_data_p()` functions are available
by defining `MRB_USE_CUSTOM_RO_DATA_P`.

(Limitation) It can not be defined as an inline function.