Compile Support for Arduino Due

Reduce string.h.

khash cleanup.

 This is a first step. It will be reduced stdlib.h in each files later.

display BasicObject information in mirb

fix segmentation fault in BasicObject#inspect

Remove bit-fields. They doesn't reduce memory on major environments.

add mrbgem: mruby-array-ext

Add break to default case in switch statement

Improve Android platform support.