Implement returning option/result of a handle in code generation

[cpetig]

Fixes #1120

[alexcrichton]

Thanks! I think additionally this can be updated to use self.emit_and_lift(ty, addr, &I32Load { offset }), like the Type::U32 case above this

[sunfishcode]

And if you'd like to, you could also implement write_to_memory for resources, using lower_and_emit, which works similarly to emit_and_lift.

[alexcrichton]

Thanks!