This PR ensures `where finally` tactics can access private data under
the module system even when the corresponding holes are in the public
scope as long as all of them are of proposition types.
This PR implements a `finally` section following a (potentially empty)
`where` block. `where ... finally` opens a tactic sequence block in
which the goals are the unassigned metavariables from the definition
body and its auxiliary definitions that arise from use of `let rec` and
`where`.
This can be useful for discharging multiple proof obligations in the
definition body by a single invocation of a tactic such as `all_goals`:
```lean
example (i j : Nat) (xs : Array Nat) (hi : i < xs.size) (hj: j < xs.size) :=
match i with
| 0 => x
| _ => xs[i]'?_ + xs[j]'?_
where x := 13
finally all_goals assumption
```
---------
Co-authored-by: Sebastian Graf <sg@lean-fro.org>
