feat: implement mvcgen?, expanding to mvcgen invariants?

[sgraf812]

This PR implements a hint tactic mvcgen?, expanding to mvcgen invariants?

Example:

/--
info: Try this:
  [apply] mvcgen invariants?
---
info: Try this:
  [apply] mvcgen [mySum] invariants?
---
info: Try this:
  [apply] mvcgen +elimLets invariants?
---
info: Try this:
  [apply] mvcgen +elimLets [mySum] invariants?
-/
#guard_msgs (info) in
theorem mySum_suggest_invariant_short (l : List Nat) : mySum l = l.sum := by
  generalize h : mySum l = r
  apply Id.of_wp_run_eq h
  mvcgen?
  mvcgen? [mySum]
  mvcgen? +elimLets
  mvcgen? +elimLets [mySum]
  all_goals admit