Add remove() to Map.

doc/site/core/map.markdown

@@ -26,7 +26,11 @@ but may be in any order, and may even change between invocations of Wren.
 Implements the [iterator protocol](../control-flow.html#the-iterator-protocol)
 for iterating over the elements in the list.
 
-### **removeAt**(key)
+### **remove**(key)
+
+Removes [key] and the value associated with it from the map. Returns the value.
+
+If the key was not present, returns `null`.
 
 ### **values**