util: Don't allow Base58 decoding of non-Base58 strings. Add Base58 tests.
Summary:
Don't allow Base58 decoding of non-Base58 strings. Add Base58 tests.
Backport of core PR17721.
Depends on D8153.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8154