util: Move util/string.h functions to util namespace

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.

src/bitcoin-cli.cpp

@@ -42,6 +42,9 @@
 #include <event2/keyvalq_struct.h>
 #include <support/events.h>
 
+using util::Join;
+using util::ToString;
+
 // The server returns time values from a mockable system clock, but it is not
 // trivial to get the mocked time from the server, nor is it needed for now, so
 // just use a plain system_clock.