Support loading/storing 8 bits

[brendandahl]

If we end up reusing the existing memory instructions and only allow i8 arrays there will be some overlap with the regular array instructions. e.g.

i32.store8 (type $i8_array) will effectively be the same as array.set

and likewise for loads

i32.load8_s (type $i8_array) will effectively be the same as array.get_s
i32.load8_u (type $i8_array) will effectively be the same as array.get_u