The concrete return type was the type of the LHS, which is wrong for
most mixed-signedness code.  We can get it right using double-dispatch,
but let's wait until we know we need to do so.

I left the comparison operators because their return type is always Bool.

We must compensate for Class.getMethod's nondeterministic ordering.

For Vec and Bundle, toBits and fromBits got the flattening order wrong
and so weren't inverses of each other.

This avoids some cases where cloneType would need to be defined.

This obviates any latent fuckery with := or <>.

This is a public API, so we mustn't change it without good reason.

We now call collectElts when the module stack is popped,
rather than having to do it all over the place.

Packagedir