Replace 0 with NULL because NULL is used in struct pointer.
Replace mrb_intern() with mrb_intern2() or mrb_intern_cstr().
Replace mrb_intern() with mrb_intern2() in random.c
Add comments in variable.c
Replace 0 with FALSE and replace 1 with TRUE.
Replace int with mrb_bool because return a value is boolean.
Replace 0 with NULL because set NULL in sturct pointer.
Merge pull request #1374 from suzukaze/refactor-vm.c

Replace 0 with NULL because NULL is used in struct pointer.
Merge pull request #1375 from suzukaze/replace-mrb_intern

Replace mrb_intern() with mrb_intern2() or mrb_intern_cstr().
Merge pull request #1376 from suzukaze/replace-mrb_intern-in-random.c

Replace mrb_intern() with mrb_intern2() in random.c
Merge pull request #1377 from suzukaze/add-comments-in-variable.c

Add comments in variable.c
Merge pull request #1378 from suzukaze/refactor2-vm.c

Replace 0 with FALSE and replace 1 with TRUE.
Merge pull request #1379 from suzukaze/refactor-gc.c

Replace 0 with NULL because set NULL in sturct pointer.