Mkr serializer

Re-implementation of mKR serializer using raptor_serialize_turtle.c and raptor_turtle_writer.c
context->emit_mkr = 1 for mkr, = 0 for turtle in raptor_serialize_turtle.c
Ditto for new emit_mkr argument for _prefix and _base procedures in raptor_turtle_writer.c
itoa?  If this is necessary (and I'm not sure), it should be static and have a symbol name starting raptor_ e.g.
```static void raptor_itoa(...)```

However given this is 2 lines of code and assigns a 1 char string, I think this is not needed.