Initial map implementation.

Still lots of methods missing and clean up and tests to do.
Also still no literal syntax.

But the core hash table code is there and working. The supported
key types are all, uh, supported.

builtin/core.wren

@@ -88,4 +88,9 @@ class List is Sequence {
   }
 }
 
+class Map {
+  // TODO: Implement this.
+  toString { "{}" }
+}
+
 class Range is Sequence {}