Extended Module#const_get to support class paths

Added Array#bsearch_index

Fixed return value of Module#include and #prepend

It reduces the memory consumption and sometimes improve the
performance as well.  For example, the consumed memory size
of `bench/bm_ao_render.rb` is reduced from 1.2GB to 1GB, and
its total execution time become 18.795 sec from 22.229 sec.

Fixed setting custom rbfiles in gemspec

Implemented Module#singleton_class?

Merge branch 'const_set_mod_to_s' of https://github.com/christopheraue/mruby into christopheraue-const_set_mod_to_s

It will no longer shrink arena region. Instead `vm.c` uses
a static function `mrb_gc_arena_shrink()` to shrink.

added tests for #3736

added basic tests for while/until/break/next