fix(library/unifier): missing set_conflict

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(library/standard): remove workarounds

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/unifier): allow computation when solving flex-rigid constraints

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