Use compile-time constants instead of unnamed enumerations

Summary:
Remove "enum hack"

Backport of core PR10749

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2454