Revamp qstrs: they now include length and hash.

Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h
diff --git a/py/objset.c b/py/objset.c
index a9ba2ad..cf4545c 100644
--- a/py/objset.c
+++ b/py/objset.c
@@ -6,7 +6,7 @@
 #include "nlr.h"
 #include "misc.h"
 #include "mpconfig.h"
-#include "mpqstr.h"
+#include "qstr.h"
 #include "obj.h"
 #include "runtime.h"
 #include "runtime0.h"