Use the new struct module's ability to pack and unpack standardized

Use the new struct module's ability to pack and unpack standardized
data formats.  The _xdr module is no longer used, since struct
supports the required IEEE floats and doubles.

(I have one doubt about not using _xdr.  The struct module doesn't
handle Inf, NaN and gradual underflow correctly.  If the _xdr module
does these things better, it may still have a (small) competitive
advantage.  On the other hand, since not all platforms support IEEE
floating point, it's not clear that it would be a good idea to ever
transfer Inf or NaNs.  Gradual underflow can be fixed in the struct
module.