refactor: Make HexStr take a span

Summary:

Make HexStr take a span of bytes, instead of an awkward pair of
templated iterators.

Backport of core PR19660.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9183