Migrate FinSet types to ACSets.jl

As of https://github.com/AlgebraicJulia/ACSets.jl/pull/32 and https://github.com/AlgebraicJulia/ACSets.jl/pull/47, we effectively have two parallel implementations of finite sets: `PartsType` in ACSets.jl and `FinSet` here in Catlab. We should consolidate them into a single type, moving `FinSet` to ACSets.jl and then eliminating `PartsType`.

This design is consistent with the intended separation between ACSets and Catlab: the basic data structures (finite sets, acsets) will be in ACSets.jl and the category-theoretic machinery, beginning with morphisms (functions between finite sets, transformations between acsets) will be in Catlab.

This should be done prior to #706.