Change CWRS indexing to use Pyramid VQ's magnitude ordering.

This lets us encode and decode directly from the pulse vector without an
 intermediate transformation.
This makes old streams undecodable.
Additionally, ncwrs_u32() has been sped up for large N by using the sliding
 recurrence from Mohorko et al.
ncwrs_u64 could be sped up in a similar manner, but would require a larger
 table of multiplicative inverses (or several 32x32->64 bit multiplies).
Note that U(N,M) is now everywhere 1/2 the value it used to be.