Made Inner and JoinWithImplicitOnClause accessible

[LucaCappelletti94]

This PR addresses discussion #4942:

• Exports JoinWithImplicitOnClause as a sealed trait
• Exports the joins::Inner marker struct

This is done in order to specify trait constraints necessary to use the inner_join method on a generic type T, which requires: T: JoinWithImplicitOnClause<Rhs, Inner>.

[LucaCappelletti94]

With this PR, it is now possible to write, using nested tuples, variadic joins as shown here.

With those, in turn, I was able to write variadic select queries of different kinds, as shown here.

[weiznich]

Given #3223 I would rather not like to expose these types as this restricts any future possibility to change the type level representation of these joins later on to fix that issue at some point.