Define type filtering through an intersection operation

[HertzDevil]

Refactors type filtering operations (used for is_a? and as) into a new internal method Crystal::Type.common_descendent, a dual to common_ancestor, in a similar vein as #10527. This is a prerequisite for eventually implementing #2404.

The first commit mostly copies the type-versus-type portion of Type#restrict to common_descendent, subsequent commits are further refactors. There is a slight performance boost here because the new intersection no longer needs a MatchContext object (the context is never needed unless AST node restrictions are involved, e.g. to access self or bind free variables). Since type restrictions must behave like AST node restrictions for the time being, the old logic in restrictions.cr remains intact.

The new method uses #implements? where the restriction code would use #restriction_of?, and once again the context object is unneeded here. There is exactly one spec that fails due to this change, and I have confirmed that it would be fixed by #10779.

This PR is purely a refactor; it does not make type filtering commutative yet, and it is relatively apparent where commutativity fails. It is up to a second PR to turn this method into a commutative greatest common descendent operation.

[asterite]

I don't know if we are in a PR freeze already, but otherwise feel free to merge this.

[HertzDevil]

5c4b946 contains #10883 and the corresponding post-refactor quick fix.