find-stretch.sage: Calculate stretch shifts for various block sizes.

This is an improved version which searches in the expected order.

For a block size w and shift c, define the domain length D(w, c) to be
the largest D such that

	x, i |-> (x || x XOR (x << c))[i..i + w]

is strongly XOR-universal over {0, 1}^w, {0, 1, ... D - 1}.

The algorithm is to choose the shift c which maximizes

  * maximizes floor(log_2(D(w, c))),
  * minimizes c mod 8, and
  * maximizes c

in that priority order.