refactor: Take Span in SetSeed

Summary:
This makes calling code less verbose and less fragile. Also, by adding
the CKey::data() member function, it is now possible to call HexStr()
with a CKey object.

This is a backport of core#23409

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17156