adjust box/c to use the val-first-projection protocol
Makefile: add `IF EXISTS` guard on `del` for a Win32 target

Fixes Windows installer builds in a fresh working directory.
fix bug introduced in 23d5cf56
tweak save-file docs based on my own earlier confusion
adjust drracket IO test to be more forgiving to IO order
add box/c tests (should have been included in 37a4a9ada6)
bring down below 102 columns
fix mangling of drracket io test
Make group-by preserve the ordering of the original list.
vectorof contract: add some tests and make flat variant subscribe to the val-first protocol
redex: fix stepper

This was exposed by the removal of the undefined value.

It's not clear if there was a bug lurking here or not.
The call to "move" was being bypassed on the initialization
of "pb", so the side effect of setting "set-visible?" wouldn't
have happened. Either that really didn't cause any problems,
or perhaps things looked strange briefly until the user did
something.
racket/unit: repairs related to signature extension

Fix signature extension so that the scope of identifiers in the
extended signature behaves the same as `open`.

Also, fix `struct` (or generally `define-values-for-export`)
in its interaction with `open` and `unit-from-context` (two
different bugs).
added non-terminal-gap-space
bring down below 102 lines
Change the semantics for _!_ variables when they are under ellipses

In particular, make the semantics less insane. There is now a
much simpler description of how these two interact (see docs)

Altho this is a backwards incompatible change, it seems relatively
unlikely to affect anyone negatively and it makes the R6RS semantics
better and makes it possible to think about enumerating expressions
from that grammar
fix bug in recent _!_ semantics change
remove unique? side-conditions from the r6rs language due to the _!_ semantics change
remove the "variable not a prefix" side-condition from the r6rs model
repair JIT bug

The bug incorrectly tracks the value that is available in virtual
register R0 --- only in the case that some value was known to be ready
in R0, and some other value was known to be ready in R1, and a value
is moved from R1 to R0.

Closes PR 14523
Added openssl, openssl/sha1, and openssl/md5.

Also, listed new libs in TR scribble docs.