fix(frontends/lean/placeholder_elaborator): missing 'return'
fix(frontends/lean/elaborator): missing register_meta
feat(frontends/lean/elaborator): use tactic_hints for unsolved placeholders
feat(frontends/lean/elaborator): discard partial solution during
class-instance resolution, use only tactic_hints associated with
classes, enforce is_strict
fix(frontends/lean/proof_qed_elaborator): must also create
metavar_closure before solving nested proof_qed

The bug was exposed by the new policy for handling class-instance
resolution. In the new policy, we reject partial solutions.
The bug fixed in this commit was being masked by a partial solution that
was being "completed" later.