Update psqueue.m code style.

library/psqueue.m:
    Update coding style.

    In min_view/1 there's two variable names are incorrect (they should be
    swapped) this doesn't change the program but may confuse people reading
    the code.

tests/hard_coded/psqueue_test.exp:
tests/hard_coded/psqueue_test.m:
    Also improve the coding style in the test code.
    Add lables to all of the test outputs.
Update psqueue.m's interface to match other library modules.

library/psqueue.m:
    Make the psqueue interface more consistent with other standard library
    modules.

tests/hard_coded/psqueue_test.exp:
tests/hard_coded/psqueue_test.m:
    Update tests.