Increase sparsity of pippenger fixed window naf representation

Summary: This is a backport of scep256k1 PR513

Test Plan:

ninja check-secp256k1
ninja bench-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4879