* MAKE-MEMBER-TYPE was doing several SUBSETP calls,
instead just walk once over the list of members.
* UNION and NUNION were using an O(N^2) implementation
for all input sizes. Use a hashtable (giving O(N))
for large inputs when :TEST is something we can
use with a hash-table.
Thanks to Damien Diederen for catching a typo in the new UNION /
NUNION code.