While it is still technically possible to test stage 0, it is not part of any
of the main testing rules and maintaining xfail-stage0 is a chore. Nobody
should worry about how tests fare in stage0.
This was previously disallowed by the typechecker and not properly handled
in trans. I removed the typechecker check (replacing it with a simpler
check that spawned functions don't have type params) and fixed trans.
Closes#756.
