Remove DEBUG macro definitions.
gc.c: Enable DEBUG(x) if GC_DEBUG is defined.
vm.c: Enable DEBUG(x) if VM_DEBUG is defined.
add test cases for GC API
Compact flags for GC
Skip sweeping old slots which don't contain any young object.
Merge pull request #701 from monaka/pr-remove-debug-macros

Remove debug macros from mruby.h
Merge pull request #703 from authorNari/skip_sweeping_old

Skip sweeping old slots which don't contain any young object in the minor GC
Merge pull request #704 from authorNari/gc_test

Add test cases for GC API
Merge pull request #705 from authorNari/compact_flags

Compact flags for GC