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.

Singleton class of frozen object should be frozen

Before this patch:

  p (class << Object.new.freeze; self end).frozen?                #=> false
  sc = class << (o=Object.new); self end; o.freeze; p sc.frozen?  #=> false

After this patch / Ruby:

  p (class << Object.new.freeze; self end).frozen?                #=> true
  sc = class << (o=Object.new); self end; o.freeze; p sc.frozen?  #=> true

Fix modiying class variable to frozen class/module

Fix "ambiguous first argument" warning in `test/t/float.rb`

Add `assert_match` and `assert_not_match`

Change modifier to `MRB_INLINE` from `static inline`

Use `MRB_ASPEC_XXX()` macro in `codedump()`

Commented out `String#scan` because it is not implemented yet

Expand `BOXWORD_SET_VALUE()` macro in `include/mruby/boxing_word.h`

In `SET_OBJ_VALUE()`, branch isn't removed because `switch` condition isn't
constant expression.