fix(library/elaborator): tag meta_app constraints of the form 'ctx |- m?[inst:i v] t1 =:= t2' as expensive

This commits also adds a new unit test that demonstrates non-termination due to this kind of constraint.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/elaborator): tag meta_app constraints of the form 'ctx |- m?[inst:i v] t1 =:= t2' as expensive