It was a workaround for pre-C++17 std::string which didn't support
passing a string_view to various functions/operators. There's only one
place left that needs an explicit conversion, and that's where it is
used as a map key; so just be explicit there and remove llarp::str()
everywhere else.
string_view was implicitly convertible to std::string, but
std::string_view is only explicitly convertible. This makes the
`operator std::string` explicit to be more compatible, and re-adds a
bunch of explicit string casts to the code where needed.
(This also fixes the build if changing the standard to c++17)
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).
