Add immutable collections hierarchy with runtime/compiler/docs/tests

docs/ImmutableSet.md

@@ -0,0 +1,34 @@
+# ImmutableSet built-in class
+
+`ImmutableSet` is an immutable set of unique elements.
+It implements [Collection] and [Iterable].
+
+## Creating
+
+    val a = ImmutableSet(1,2,3)
+    val b = Set(1,2,3).toImmutable()
+    val c = [1,2,3].toImmutableSet
+    >>> void
+
+## Converting
+
+    val i = ImmutableSet(1,2,3)
+    val m = i.toMutable()
+    m += 4
+    assertEquals( ImmutableSet(1,2,3), i )
+    assertEquals( Set(1,2,3,4), m )
+    >>> void
+
+## Members
+
+| name          | meaning                                             |
+|---------------|-----------------------------------------------------|
+| `size`        | number of elements                                  |
+| `contains(x)` | membership test                                     |
+| `+`, `union`  | union, returns new immutable set                    |
+| `-`, `subtract` | subtraction, returns new immutable set            |
+| `*`, `intersect` | intersection, returns new immutable set         |
+| `toMutable()` | create mutable copy                                 |
+
+[Collection]: Collection.md
+[Iterable]: Iterable.md