1.0.9.1: faster MAKE-MEMBER-TYPE, UNION, and NUNION

* 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.