Get rid of namespace imports & backporting univalue
Summary: Remove namespace imports as they pollute namespace and are prone to symbol clashes. Backport univalue to latest version, which also includes namespace removals.
Test Plan: These changes build. No new tests should be necessary.
Reviewers: deadalnix, O1 Bitcoin ABC, #bitcoin_abc
Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc
Maniphest Tasks: T6
Differential Revision: https://reviews.bitcoinabc.org/D647