Beefed up the pqueue implementation.

library/pqueue.m:
    Added predicates is_empty/1, peek/3, peek_key/2, peek_value/2,
    det_peek/3, merge/3.  Added functions det_peek_key/1, det_peek_value/1.

    Also reformatted code to fit within 76 columns.
Refine Micheal's pqueue changes

library/pqueue.m:
    Refine Micheal's changes where necessary to conform to our style.

NEWS:
    Updated news file.