util: Move util/string.h functions to util namespace
Summary:
There are no changes to behavior. Changes in this commit are all additions, and
are easiest to review using "git diff -U0 --word-diff-regex=." options.
Motivation for this change is to keep util functions with really generic names
like "Split" and "Join" out of the global namespace so it is easier to see
where these functions are defined, and so they don't interfere with function
overloading, especially since the util library is a dependency of the kernel
library and intended to be used with external code.
This is a partial backport of core#29015
https://github.com/bitcoin/bitcoin/pull/29015/changes/4f74c59334d496f28e1a5c0d84c412f9020b366f
Depends on D19754
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D19755