EFUN: mapping_contains

SYNTAX:
int mapping_contains( result, map, key );
  mixed &result | mapping result;
  mixed &map | mapping map;
  mixed key;

DESCRIPTION:
Returns 1 if key is in map. result contains the value associated with the key.
Note that a reference to result has to be passed to mapping_contains, that's
what the & is good for. The result won't be overwritten if the mapping doesn't
contain a value for the index.
