Allow values that aren't exactly 8 bytes

[iafisher]

Right now the code generator does everything with 8-byte registers and stack locations. This is inefficient in a couple of ways:

• For values that fit in fewer bytes, like booleans, it wastes a lot of space.
• Values that are larger than 8 bytes must be heap-allocated even if they are small enough to fit comfortably on the stack, e.g. strings which have an 8 byte length and an 8 byte pointer. This requires both an extra heap allocation (on top of the necessary allocation for the actual string data) and an extra layer of indirection whenever the value is used.

On the other hand, storing everything as 8 bytes makes both the code generator and the runtime library simpler (the latter mainly because it's easier to implement a generic list in C if every value has the same type).