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.
Fix it-never-happened performance regression.

Diligent reviewer observed that a hot spot was possibly
being made hotter. Reviewer's suggested remedy was a
spectacular bust, but studious observation revealed the news
lash that expensive methods are expensive and we should
avoid calling them if we can.

Put short-circuit test back in front of unapply call.
Now the time spent in unapply is within a few percent.
Merge pull request #2215 from paulp/issue/7228

SI-7228, bug in subtyping.
Restore sketchy dependency to quick.bin.

Apparently we cannot run at all without the continuations
classes on the classpath. At least that it was I conclude
from the fact that quick/bin/scala hasn't worked since
I routed the plugin classes into continuations-library.

This restores them to quick's classpath so we can have
qscala again.
Merge pull request #2254 from paulp/pr/buildxml

Restore sketchy dependency to quick.bin.