feat: add MapEntry class and make Map a Sequence for direct iteration

Add MapEntry class to expose key-value pairs during map iteration, and make Map extend Sequence so maps can be directly iterated with `for` loops. Rename the internal `iterate_` primitive to `iterate` and update error messages for invalid map iterators.

test/core/map_entry/new.wren

@@ -0,0 +1,4 @@
+var entry = MapEntry.new("key", "value")
+
+System.print(entry.key)   // expect: key
+System.print(entry.value) // expect: value