We now (try to) postpone `match ... with` elaboration when pattern
variables and patterns still contain metavariables before invoking `mkMatcher`.
This improvement makes sure we can elaborate an example submitted by
Daniel Selsam.
Remark: this update may create performance problems since we
backtrack `elabMatchTypeAndDiscrs` and `elabMatchAltView`.
We hope this is not a problem in practice since we use the "quick-check"
`waitExpectedTypeAndDiscrs`. Recall that we could compile Lean
without this commit. So, it suggests cases where we need to postpone
after `elabMatchAltView` are rare.
