Adds a TrimWhiteSpace instead of using abseil's.
Adds Catch2 tests for it, and also converts the existing str tests to
catch (which look much, much nicer than the gtest ones).
The comparison done here was really weird: by comparing lengths *before*
contents "zz" would sort before "aaa". It wasn't invalid for the
specific purpose being used here (looking for true/false values), but
would be highly broken if someone tried to use it elsewhere.
Also renamed it because it really is just a `<` implementation, not a
full cmp implementation.
