Background: lstrip and rstrip were broken by changes to String in:
0e29f7974b59e4440cf02e1388fb9d8ab2b5c5fd
which removed it's access to Vector::find(CharType).
Moved Vector's find up into CowData so it can be shared by Vector and String.
Added String::find_char using CowData::find.
Implemented rstrip and lstrip using find_char.
Added a few tests for String rstrip and lstrip.