Implement the 'add' builtin promised by the docs' examples.

testdata

@@ -211,6 +211,10 @@ null
 null
 [1,2,3,4]
 
+map(add)
+[[], [1,2,3], ["a","b","c"], [[3],[4,5],[6]], [{"a":1}, {"b":2}, {"a":3}]]
+[null, 6, "abc", [3,4,5,6], {"a":3, "b": 2}]
+
 #
 # User-defined functions
 # Oh god.