fix(library/unifier): more bugs in the unifier

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(tests/lean/run): another test for the unifier

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): fix cryptic error message

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/justification): duplicate position

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>