Include mruby.h. As KHASH_DEFAULT_SIZE might be defined in mrbconf.h.
Remove unnecessory stdint.h.
Remove stdlib.h from mruby.h. It is for portability (care for freestanding environments).
 This is a first step. It will be reduced stdlib.h in each files later.
Remove stdio.h as it is included in mrbconf.h.
resolve conflict from #1017