Emitter: So...maybe tuples can't use that setup_sigs_for_build thing.

Because what's within the tuple may want to lay down some signatures or
call the generics. So process tuples by unrolling signatures one at a time if
possible. Then do a second pass and fill in the signatures to build the right
tuple signature.

A test has been added which didn't work before this patch.
Emitter: Calls should be careful to not double-resolve a signature.