Rework memory type hierarchy

[bchurchill]

I think dealing with all the memory operands would be way easier if we just encoded the length (in bytes) into the Operand's val2_ field. It would mean we won't have casts to/from M8 everywhere and we could remove a ton of these switch statements over the type() field which can't be all that fast to begin with.

I don't want to do this now since the develop branch isn't all that stable at the moment.