No changes in logic so far, just simple refactoring

Add `MapOps` class with the following signature:

  public class MapOps<K>(compare : (K,K) -> O.Order) {

    public func put<V>(rbMap : Map<K, V>, key : K, value : V) : Map<K, V>

    public func fromIter<V>(i : I.Iter<(K,V)>) : Map<K, V>

    public func replace<V>(rbMap : Map<K, V>, key : K, value : V) : (Map<K,V>, ?V)

    public func mapFilter<V1, V2>(f : (K, V1) -> ?V2, rbMap : Map<K, V1>) : Map<K, V2>

    public func get<V>(key : K, rbMap : Map<K, V>) : ?V

    public func delete<V>(rbMap : Map<K, V>, key : K) : Map<K, V>

    public func remove< V>(rbMap : Map<K, V>, key : K) : (Map<K,V>, ?V)

  };

The other functionality provided as standalone functions, as they
don't require comparator:

  public type Direction = { #fwd; #bwd };

  public func iter<K, V>(rbMap : Map<K, V>, direction : Direction) : I.Iter<(K, V)>

  public func entries<K, V>(m : Map<K, V>) : I.Iter<(K, V)>

  public func keys<K, V>(m : Map<K, V>, direction : Direction) : I.Iter<K>

  public func vals<K, V>(m : Map<K, V>, direction : Direction) : I.Iter<V>

  public func map<K, V1, V2>(f : (K, V1) -> V2, rbMap : Map<K, V1>) : Map<K, V2>

  public func size<K, V>(t : Map<K, V>) : Nat

  public func foldLeft<Key, Value, Accum>(
    combine : (Key, Value, Accum) -> Accum,
    base : Accum,
    rbMap : Map<Key, Value>
  ) : Accum

  And foldRight with the same signature as foldLeft

The following functions are new for the API:
- MapOps.put, MapOps.delete
- MapOps.fromIter, entries, keys, vals
- MapOps.mapFilter, map
- foldLeft, foldRight

Problem: now order is not consistent within new module and with old
modules as well.

Solution: make the map argument always go first

In addition to tests this patch removes `direction`
argument from `keys` and `values` function to keep
them simple and provides a new function `Map.empty`
to create a map without knowing its internal representation.

* rename `rbMap` into `m` in signature for brevity & consistent language
* rename `rbMap` into `map` in examples for brevity & encapsulation sake
* rename `tree` into `map` in doc comments for the encapsulation sake