Use sliced to remove the sighash byte from a signature to check for low S

Summary:
This removes unecessary copies.

Depends on D1571

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1572