fix(library/unifier): simple bug

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add 'classical' module

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/unifier): delay constraints where the lhs or rhs are of the form (?m ... (intro ...))

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/unifier): divide constraints in clear groups

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/unifier): allow unifier to unfold opaque definitions of the current module

It is not clear whether this is a good idea or not. In some cases, it seems to do more harm than good.

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