Typecheck: Do more substitution on reduced preds

Add a test case that fails to typecheck without the substitution.
It's unclear if this is the most efficient fix -- for example, if the
substitution is just covering for an issue earlier where the
predicates are created -- but this doesn't seem to increase the run
time of the testsuite, so it's probably ok.

src/comp/TCMisc.hs

@@ -423,8 +423,14 @@ satMany dvs es rs_accum bs s (p:ps) = do
             -- We also accum the bindings to get from "p" to "needed".
             -- we keep growing the substitution because anything we
             -- have learned we can commit to
+            --
+            -- We apply the substitution to the "needed" preds because we
+            -- encounted an example where they were unsubstituted and that
+            -- prevented satisfying -- is this apSub just covering for an
+            -- issue elsewhere (that should not have returned unsubst'd preds)?
+            --
             rtrace ("satMany Right: " ++ ppReadable needed) $
-            satMany dvs es (needed ++ rs_accum) (bs'++bs) (s' @@ s) ps
+            satMany dvs es ((apSub s' needed) ++ rs_accum) (bs'++bs) (s' @@ s) ps
         ([], bs', s') ->
             -- If p is satisfied, we "drop" it, but add its binding and
             -- substitution.