bug#46722: test-map-into fails

[dick]

You can't do,

(funcall
 (funcall
  (backquote
   (lambda (arg)
     (lambda () arg)))
  t))

which is what `cl--generic-get-dispatcher` tries to do.  You need an
apply-partially somewhere in there.

Also in same function, `method-cache` buys you nothing, so getting rid of
it along with `with-memoization` and `(require subr-x)` is a not
insubstantial win.

Fixing may or may not put the kibosh on the heisen-flap.