Make Set and Map finite, add ISet and IMap.

[jaylorch]

Set<A> and Map<A> now represent finite sets and maps, respectively. ISet<A> and IMap<A> are the new names for the old-style possibly-infinite sets and maps.

This new design is motivated because (1) it allows recursive types, such as having an enum T with a Set<T> field; and (2) finite sets are quite common in user code, and it's easy to go down a wild goose chase trying to get an ambient broadcast property to instantiate that turns out to be a missing .finite().

The simplest porting path for existing code is to use the infinite versions, which work like the previous single Set and Map. Replace Set with ISet, Map with IMap, set! with iset!, and map! with imap!.

To exploit the finite sets, use the Set and Map types. Where you might have specified a set domain with a predicate (boolean closure) before, now you might start with a finite constructor and transform it with map or filter.

let evens_lt_100 = Set::new(|x| 0 <= x < 100 && x%2==0); // old predicate constructor; now produces an `Option<Set<int>>` that you demonstrate is `Some` by proving it's finite.
let evens_lt_100 = Set::int_range(0, 100).filter(|x| x % 2 == 0); // new constructor starts with finite int range

let trees = Set::int_range(5).map(|x| tree_constructor(x)); // Construct a set of trees, one per int [0,5)

Or, even better, use the recently added set_build! macro to produce a finite set. To produce a finite map, construct the finite domain as a Set and use it as the first parameter to the new Map::new.

This PR addresses issue #1512.

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[jaylorch]

Oh, I just realized I haven't made it possible to have a recursive type A using Map<A, B>. Converting to draft....