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
code style is now 'a = b' with spaces around =.  I know older code doesn't match this, but new code should