Intermediate construction typeclasses

Stream: general

[Yaël Dillies]

docs#boolean_algebra.core, docs#complete_semilattice_Inf, docs#complete_semilattice_Sup all are typeclasses which don't have any mathematical content, as any instance of them can be promoted to docs#boolean_algebra, docs#complete_lattice respectively. They are only used for intermediate constructions, a role which today is filled by constructors.
What do you think of getting rid of them in favor of new constructors?

[Yury G. Kudryashov]

complete_semilattice_Sup used to be a custom constructor.

[Yury G. Kudryashov]

@Scott Morrison introduced intermediate classes in #6797