`test-covered' to use just the expression -- looks like there's no
reason to use an additional key.
Also, change its uses to map each syntax to an mcons where its mcar is
used to track coverage. This is done everywhere, since it turns out to
be much faster to insert a `set-mcar!' with a 3d mpair, rather than a
call to a thunk.
Note that it still uses mpairs as a hack. It "works" in the same way
that this simplified example does:
(define-syntax m
(let ([b (mcons 0 0)])
(lambda (stx)
(with-syntax ([b b])
#'(case-lambda [() (mcar b)]
[(x) (set-mcar! b x)])))))
I think that it's fragile, and likely to stop working at some point, but
I don't see anything better for now.
