This did not server any real purpose and also made it too difficult to add
asserts for string memory overrwrites.

Moved all functionallity from Static_binary_string to Binary_string.

Other things:
- Added asserts to q_xxx and qs_xxx functions to check for memory overruns
- Fixed wrong test in String_buffer::set_buffer_if_not_allocated().
  The idea is to reuse allocated buffers (to avoid extra allocs), which
  the code did not do.