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.

test/map/count.wren

@@ -0,0 +1,12 @@
+var map = new Map
+IO.print(map.count) // expect: 0
+map["one"] = "value"
+IO.print(map.count) // expect: 1
+map["two"] = "value"
+IO.print(map.count) // expect: 2
+map["three"] = "value"
+IO.print(map.count) // expect: 3
+
+// Adding existing key does not increase count.
+map["two"] = "new value"
+IO.print(map.count) // expect: 3