keys() - return an array of the keys from the (key, value) pairs in a
mapping
mixed *keys( mapping m );
keys() returns an array of keys (indices) corresponding to the keys in
the (key, value) pairs stored in the mapping m.
For example, if:
mapping m;
m = (["hp" : 35, "sp" : 42, "mass" : 100]);
then
keys(m) == ({"hp", "sp", "mass"})
Note: the keys will not be returned in any apparent order. However,
they will be returned in the same order as the corresponding values
(returned by the values() efun).