minor edits
unXFAIL ask(Q.real(NaN)) is None tests

Note: the old assumptions return None for queries about NaN whereas
the new assumptions (because NaN gets handled by _number) returns
False -- if it is important that this return None, the following can
be added to the pertinent handlers:

    NaN = staticmethod(lambda x, y: None)
sort assumptions dict when printing
more granular separation of re and im parts of Pow
modify old assumptions for Pow
modification of new assumptions
do not conclude if there is no precision