add a new `WithSubst tm env to` record that packages a `tm from`
with a `Subst env from to`, and write instances for just that. the
rest of the AST can be derived
e.g. "coe [_ ⇒ A] @p @q s" should immediately reduce to "s",
but if the "_ ⇒ A" happened to use an SY it didn't.
this will still happen if a wrong SY sneaks in but the alternative is
re-traversing the term over and over every time whnf runs
