Revert "Get rid of namespace imports & backporting univalue"
Summary: This reverts commit 41abbec5fd78a59b020a5130a6e0833ff024ba25.
Test Plan: Actually build.
Reviewers: schancel, freetrader, #bitcoin_abc, dagurval
Reviewed By: #bitcoin_abc, dagurval
Differential Revision: https://reviews.bitcoinabc.org/D731