Small String Optimized String

The Small String Optimized (SSO) string data-type attempts to reduce the number of heap allocations that happen at runtime. If a string is less than 23 bytes on 64bit it can be stored directly in the object, avoiding a dynamic allocation. There exist two modes for each String, a short string - and a long string mode in which the dynamic memory is managed using Nim's destructors. The implementation is based on clang's std::string class. This technique might improve runtime performance and reduce fragmented memory where this is most needed i.e. on embedded systems.