Fix element and parent classes of Hom categories to be abstract, and simplify the Hom logic.

[nthiery]

This patch fixes the parent and element classes for Hom categories to be purely abstract, and simplifies the Hom logic:

• Unified the logic for selecting the class when building a Homset (e.g. Homset, RingHomset?, HeckeModuleHomspace?, ...). This is now systematically done through the _Hom_ hook. The logic still has a fundamental flaw, but that's for the later #10668.
• The cache for Hom is handled at a single point in Hom In particular, homsets created via the _Hom_ hook are now unique.
• If category is None, Hom simply calls itself with the meet of the categories of the parent, which removes a cache handling duplication.
• Parent.Hom calls Hom directly (removes duplicate _Hom_ logic).
• ParentWithBase?.Hom was redundant and is gone.
• Reduce the footprint of the current trick to delegate Hom(F,F)(on_basis=...) to module_morphism, allow for the diagonal option too, an make sure the homset category is set properly.
• Update a doctest in sage.modules.vector_space_homspace to take into account that homsets created via _Hom_ are now unique.
• Scheme is (apparently) an abstract base class; so it should not be instantiated. I changed some doctests in sage.schemes.generic.SchemeMorphism? to use instead the concrete Spec(ZZ). Those doctests were breaking because Scheme does not implement equality, which is required for Hom caching.

As a byproduct, the HeckeModules? category does not import any more HeckeModulesHomspace?, which was a recurrent source of import loops.

#11935 depends on this ticket

[nthiery]

Note: this might depend on #12875

[nthiery]

Note: there are two doctests failures that I don't know how to handle, related to the refcounting of Singular rings::

sage -t  devel/sage-combinat/sage/rings/polynomial/multi_polynomial_libsingular.pyx
**********************************************************************
File "/opt/sage-5.0.beta11/devel/sage-combinat/sage/rings/polynomial/multi_polynomial_libsingular.pyx", line 418:
    sage: len(ring_refcount_dict) == n
Expected:
    True
Got:
    False
sage -t  devel/sage-combinat/sage/libs/singular/ring.pyx
**********************************************************************
File "/opt/sage-5.0.beta11/devel/sage-combinat/sage/libs/singular/ring.pyx", line 469:
    sage: ring_ptr in ring_refcount_dict
Expected:
    False
Got:
    True

Help fixing those welcome!

[nthiery]

The updated patch includes two small related improvements I had in later patches. I moved them here to resolve the conflict.