Use standard names for FinSet and FinRel

[epatters]

Resolves #215 and follows up #184/#197. In summary:

• FinOrd and FinOrdFunction are renamed to FinSet and FinFunction
• FinOrdRelOb and FinOrdRelation are renamed to FinRel and FinRelation
• The abstract type FinFunction has concrete subtypes FinFunctionCallable and FinFunctionVector and FinRelation has subtypes FinRelationCallable and FinRelationMatrix. The concrete types are exported but should rarely be needed in user code, which can use the constructors for FinFunction and FinRelation
• Type parameters are used to distinguished the usual categories from their skeleta. So, FinSet{Int} is the skeleton of FinSet and FinSet{<:AbstractSet} is FinSet. Currently, only the former is implemented but the latter could be straightforwardly implemented along these lines.

I really hope this is the last time we need to rewrite this interface, so if you have concerns, speak now or forever hold your peace :)