fix(library/elaborator): in optimization for metavariable free terms

The optimization was incorrect if the term indirectly contained a metavariable.
It could happen if the term contained a free variable that was assigned in the context to a term containing a metavariable.

This commit also adds a new test that exposes the problem.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/elaborator): in optimization for metavariable free terms