Add a 'keys' function. Fixes #4.

testdata

@@ -223,6 +223,10 @@ null
 [[], {}, [1,2], {"a":42}, "asdf"]
 [0, 0, 2, 1, 4]
 
+map(keys)
+[{}, {"abcd":1,"abc":2,"abcde":3}, {"x":1, "z": 3, "y":2}]
+[[], ["abc","abcd","abcde"], ["x","y","z"]]
+
 [1,2,empty,3,empty,4]
 null
 [1,2,3,4]