Remove numpy dtypes from ArgExtFunction and SegmentedFunction

This change removes numpy dtypes from ArgExtFunction and SegmentedFunction reduction operations. These can't be used since they are unpickleable.

This change adds a make_tuple() function to loopy and uses tuples to produce neutral elements for tuple-typed reductions. As a result, these reduction operations don't need to keep a reference to their types anymore.

Closes #80 (closed)