Remove memmove(). Use value_move() instead.

value_move() is type-safer than memmove().

And may be faster on some targets.
(If it makes slow on your target,
rewrite to call memset(s1, s2, sizeof(mrb_value)*n)
in value_move().)
Merge pull request #1023 from monaka/pr-remove-memmove-from-vm.c

Remove memmove() in src/vm.c.