Change on-disk representation of NUMERIC datatype so that the sign_dscale

Change on-disk representation of NUMERIC datatype so that the sign_dscale
word comes before the weight instead of after.  This will allow future
binary-compatible extension of the representation to support compact formats,
as discussed on pgsql-hackers around 2007/06/18.  The reason to do it now is
that we've already pretty well broken any chance of simple in-place upgrade
from 8.2 to 8.3, but it's possible that 8.3 to 8.4 (or whenever we get around
to squeezing NUMERIC) could otherwise be data-compatible.