Fix implicit integer sign changes in strencodings and reduce size of strencodings decode tables

Summary:
This is a backport of core#23756

Depends on D12812

Test Plan:
With UBSAN
ninja && ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12813