refactor string parsing

Add numeric format function

fix the type of argument named 'aspec'.

'aspec' should be large at least 24 bit.

Remove memmove() in src/vm.c.

fix the type of bit field variable.

Removed unused expr

Fix a warning happens on MRB_INT64 enabled.

if 'int' type is 16bit integer, 'unsigned int flags: 21' is too large.

remove bit-shift operation.

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().)