Relax the temporary same-instance copying limitation

[lukewagner]

This PR relaxes the temporary limitation that same-instance stream/future reads/writes trap if the element type is not empty to also allow the integral and floating-point number types. The reason for the limitation is that, for more-complex types, specifying and implementing the exact behavior in the case where the source and destination linear-memory overlap (which is only possible when the readable and writable ends are same-instance) is pretty tricky. However, @dicej points out that the behavior is still quite simple when the element types are plain copyable numbers (it's as-if you read all elements and then wrote all elements, i.e., memmove, not memcpy), and allowing these (esp. stream<u8>) will address a common use case. (FWIW, lazy-lowering avoids the complexity for the complex types and makes it easy to remove the trap.) I'll add a .wast test once there's an impl draft I can test on.