Issue 4798/2: Prepare override/revert to deal with numeric subkeys

Issue 4798/2: Prepare override/revert to deal with numeric subkeys

Those are fairly cosmetic changes (partly comments) to prepare for the
possibility of using override/revert with numeric subkeys while
retaining the necessity of using a symbol as the main property in a
Context.Grob.property.subproperties... n-tuple since the organization of
an "nalist" (an alist stack containing reversible subproperty overrides)
relies on a few special key values: currently pairs and booleans are
specially interpreted while the current implementation requires an
eq?-comparable first index for efficiency reasons.