Improve pointer type annotations for loads/stores

[simonlindholm]

We currently emit e.g. *(void*)0x1234 = 12.34f;. Naively this is not too hard to fix by changing Type.ptr() to Type.ptr(Type.f32()), but #79 may cause problems, and the pointer may point to a struct, making f32* incorrect.

Perhaps we should focus on just the silly-looking cases like this and not propagate type information further.