Add byte and short types

[Hawkbat]

The types short, ushort, byte, and sbyte should be added, in order to facilitate memory-constrained operations (such as emulating an 8-bit CPU).

As WASM does not currently support signed i8 or i16 arithmetic, the types would need to be restricted to being used in struct fields only, or extra truncation operations would need to be generated during operations. I haven't decided yet if the truncation is worthwhile.