SI-7228, bug in weak subtyping.

Another in the category of bugs which involve narrowing,
widening, mediuming, dealiasing, weakening, normalizing,
denormalizing, supernormalizing, subnormalizing, and
double-bounded supersubnormalizing.

This is probably not the ideal fix, but it is an improvement.
Simplified the widening logic.

Should speak for itself.

Whenever someone changed @switch from an error to a warning,
it broke all the tests which depended on the error. I added
-Xfatal-warnings to a couple which needed it. And one of those
tests was then failing, as it must now since we couldn't get away
with what was being attempted, so I moved it to pending.
Moved some numeric subtyping logic closer to center.

Fixed bug in numeric widening related to continuations,
which enabled simplifying isNumericSubType.