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)

Remove duplicate test for mruby-string-utf8

tests for "case" expression.

adjust proc->target_class according to surrounding proc->target_class; based on a patch from @Fleurer; close #1627

Test for raise in ensure

Reduce unnecessary mrbgems full test rebuild.