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/tbf/TbfTest.err b/tests/tbf/TbfTest.err
index ad0b69b..5f49395 100644
--- a/tests/tbf/TbfTest.err
+++ b/tests/tbf/TbfTest.err
@@ -2764,6 +2764,7 @@
 +++++++++++++++++++++++++ RX : Uplink Control Block +++++++++++++++++++++++++
 ------------------------- RX : Uplink Control Block -------------------------
 RX: [PCU <- BTS] TBF(TFI=0 TLLI=0xf1223344 DIR=DL STATE=FINISHED) Packet Downlink Ack/Nack
+Got GPRS DL ACK bitmap: SSN: 0, BSN 0 to 28 - 1 (28 blocks), "RRRRRRRRRRRRRRRRRRRRRRRRRRRR"
 TBF(TFI=0 TLLI=0xf1223344 DIR=DL STATE=FINISHED) downlink acknowledge
 - Final ACK received.
 TBF(TFI=0 TLLI=0xf1223344 DIR=DL STATE=FINISHED) changes state from FINISHED to WAIT RELEASE