Use appropriate (cheaper) conversion.
First version of trivstring class (bug #9336)

As discused on the list. This is not used yet, but it is intended to provide
thread-safe read-access without the need for synchronization if the used STL
implementation does not provide it for std::basic_string. This is the case for
all implementations using copy-on-write.
Make trivstring class ready for use

The interface is now 100% unit tested, and the typedefs depend on the new
STD_STRING_USES_COW configuration variable. The only missing bit is to detect
clang and disable STD_STRING_USES_COW for clang.