By calculating the abi size of the struct, we move the stack
pointer and store each field depending on its size (i.e. a 1-byte field will use i32.store8).
This commit adds all required opcodes to perform those stores and loads.

This also gets rid of `mir_offset` as we now save results of binary operations into
locals and emit its result onto the stack within condbr instead. This makes everything a lot
simpler but also more robust.
In the future, we could look into an algorithm to re-use such locals.

For struct fields we use the new `local_with_offset` tag. This stores the struct's
stack pointer as well as the field's offset from that stack pointer.

`allocLocal` will now always allocate a single local, using a given type.