Refactor FinSets from FinSetImpls, etc

"It's something unpredictable//But in the end it's right..."
--Billie Joe Armstrong

It's too hard to have `FinDomFunction` and `FinSet` be abstract types. Make them and others concrete types according to the general design

```julia
abstract type FinSetImpl
struct FinSet
theSet::FinSetImpl
end
```

Then we refactor so that the majority of functions involving sets and functions act on the pseudo-concrete wrapper types, which means you can't access details of the implementation, but on the other hand you can map over something without constantly freaking out over whether it's a collection of `FinDomFunctionDict`s or `FinDomFunctionVector`s.