I tried to pay attention to what was actually being tested so, e.g. when I
test was just using a vec as a boxed thing, I converted to boxed ints, etc.
Haven't converted the macro tests yet. Not sure what to do there.
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.
Previously, if you wrote
let @vec[int] foo = @[];
that would be a type error. That didn't seem right, so I changed
pushdown to unify the inner type in an unop application with the
argument type of the operator type.
