Changed the lookup algorithm again, based on Reimer Behrends's post.

Changed the lookup algorithm again, based on Reimer Behrends's post.
The table size is now constrained to be a power of two, and we use a
variable increment based on GF(2^n)-{0} (not that I have the faintest
idea what that is :-) which helps avoid the expensive '%' operation.

Some of the entries in the table of polynomials have been modified
according to a post by Tim Peters.