add stronger to syntax/c
add stronger to is-a?/c
add stronger to implementation?/c and subclass?/c

also, bring down below 102 cols
add stronger for parametric->/c

(and test cases for stronger with new-∀/c)
Jack Clay: use concatenate instead of juxtapose, better for students
meta/pkg-push: note on cleaning out old package builds

I didn't implement an automatic clean-up, since this service is
going away soon, hopefully.
make installer: prevent pollution from local to catalog-based build
racket/gui win32: enable clipping-problem workaround for 32-bit mode

The specialization to Win64 to work around a Cairo clipping
problem probably had more to do with the version of Cairo,
instead of the variant of Windows, since both variants need the
workarounds after a Cairo upgrade.
racket/gui: use cairo_paint() to copy buffer to screen

Following 54e42664df, use cairo_paint() instead of cairo_fill()
to copy offscreen content onscreen, just in case it's faster.
Fix set docs.

Closes PR 14749.
Add slides to the RacketCon website.
More accurate captions.
racket/draw Windows: patch Cairo for clipped DC surface creation
racket/gui Windows: rely on patched Cairo instead of workaround
add hash/c stronger

also, bring down below 102 columns
racket/place: fix nested-place termination
Windows: fix handling of junctions as links

On Windows, a "soft link" or "junction" is different from a
"symbolic link". The current Windows documentation is
incomplete in that it describes the behavior of GetFileAttributesEx
for a symbolic link, but not for a junction, and I guessed wrong.
For consistency, junctions need to be treated like symbolic links.
racket/gui: fix reporting of numpad Enter as 'numpad-enter

On Unix and Mac OS X, the key has long been reported as #\u3,
even though the documentation says 'numpad-enter.

A more compatible change would be to adjust the documentation and
also change DrRacket and the framework to recognize #\u3 instead
of 'numpad-enter. But there doesn't seem to be a good reason to
treat #\u3 as "numpad Enter" --- particularly as opposed to treating
it as "Ctl-C" --- other than ancient Mac history.

On Windows, numpad Enter continues to be reported as #\return,
since Windows event handling doesn't naturally or easily distinguish
the two keys.
DrRacket: drop numpad Enter treatment as force-evaluation

DrRacket's interactions window tried to treat numpad Enter the same
as ctl-Return: send input to the reader, even if an S-expression is
incomplete. Since numpad Enter events were not reported as
'numpad-enter, however, the special treatment didn't take effect.

Meanwhile, it appears that some keyboard configurations report a
numpad Enter event when NumLock is on and the regular Enter key
is pressed, and that's probably not what a user wants. And
numpad Enter has never been reported differently from regular
Enter on Windows.

So, it's simpler and more consistent to just treat numpad Enter
and regular Enter the same.
fix for/fold indentation

inspired by Greg's RacketCon talk

also, Rackety