The function mrb_str_format is only defined when the mrbgem mruby-sprintf was
activated. That might not always be the case (by default it currently is).
mrb_str_format still has external linkage, so from now on mrbgem authors
have to declare it in their source code if they want or need to call it.

more accurate backtrace lineno

clone Class/Module rightly

fix utf8 codepage

escape non-ascii characters correctly in String#inspect

Increase test coverage of mruby-struct

Added rewrite of Array#[] to mruby-array-ext gem

String#length should be an alias of String#size

with Ranges (as a[1..-2])

add operator "!~".

fix ISO reference number of String#=~.

string.c: remove str_mod_check

add String#reverse, String#reverse! for UTF-8

remove superfluous includes

Clang 3.4 emits '-Wunused-function' - and it's really unused!
But according to the description this seems to be a bug:

>Warn whenever a static function is declared but not defined or
>a non-inline static function is unused. This warning is enabled by
-Wall.

Source: <http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html>

- reduce compile time by a little bit (full-core: ~0.7s for me)
- thanks to 'include-what-you-use' for some help
- include Standard C header files before any other (coding style)