Improvements to dns.rdata.to_wire().

Improvements to dns.rdata.to_wire().

This does two things.

1) Makes the file parameter optional, removing the requirement that
callers create an io.BytesIO object if all they want is the bytes.  This
is done by renaming all of the rdata subclass to_wire() methods to
_to_wire(), and making dns.rdata.to_wire() create the io.BytesIO
internally if needed, and then delegate to the subclass.

2) Add canonicalize=False parameter, allowing code reuse for conversion
to wire format and conversion to hashable format.  This also removes all
of the rdata subclass to_digestable routines that are no longer needed,
as dns.rdata.to_digestable will work for all rdata types.