tbf: Use bitvec based window methods for GPRS

Currently the old fixed 64 bit RBB based implementation is used for
GPRS.

Use the new bitvec based methods instead.

Sponsored-by: On-Waves ehf
diff --git a/tests/types/TypesTest.ok b/tests/types/TypesTest.ok
index 6ca2717..cb40d39 100644
--- a/tests/types/TypesTest.ok
+++ b/tests/types/TypesTest.ok
@@ -4,4 +4,5 @@
 rbb: 00 00 00 00 00 00 00 03 
 rbb: 00 00 00 00 00 00 00 31 
 rbb: 10 00 00 00 00 00 00 01 
-show_rbb: IIIIIIIIIIIIIIIIIIIIIIIIIIIIIRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR
+show_rbb: RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR
+show_rbb: IIRRIIIR