Key and value iteration for maps. Also toString.

test/map/key_iterate.wren

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+var a = {"one": 1, "two": 2, "three": 3, "four": 4}.keys
+
+// The precise numeric values aren't defined since they are indexes into the
+// entry table and the hashing process isn't specified. So we just validate
+// what we can assume about them.
+
+IO.print(a.iterate(null) is Num) // expect: true
+IO.print(a.iterate(null) >= 0) // expect: true
+
+IO.print(a.iterate(0) is Num) // expect: true
+IO.print(a.iterate(0) > 0) // expect: true
+IO.print(a.iterate(1) is Num) // expect: true
+IO.print(a.iterate(1) > 0) // expect: true
+IO.print(a.iterate(2) is Num) // expect: true
+IO.print(a.iterate(2) > 0) // expect: true
+IO.print(a.iterate(3) is Num) // expect: true
+IO.print(a.iterate(3) > 0) // expect: true
+
+var previous = -1
+var iterator = a.iterate(null)
+while (iterator) {
+  IO.print(iterator > previous)
+  IO.print(iterator is Num)
+  previous = iterator
+  iterator = a.iterate(iterator)
+}
+// First entry:
+// expect: true
+// expect: true
+// Second entry:
+// expect: true
+// expect: true
+// Third entry:
+// expect: true
+// expect: true
+// Fourth entry:
+// expect: true
+// expect: true
+
+// Out of bounds.
+IO.print(a.iterate(16)) // expect: false
+IO.print(a.iterate(-1)) // expect: false
+
+// Nothing to iterate in an empty map.
+IO.print({}.keys.iterate(null)) // expect: false