library: Initial work on GPRS RLC/MAC encoder/decoder

For Downlink and Uplink RLC/MAC Control blocks this is already working
quite nicely.  Data blocks is not working, as their encoding cannot be
expressed in TTCN-3 RAW syntax, and a mixture of C++/native and
RAW-generated coder will be required.
diff --git a/library/Osmocom_Types.ttcn b/library/Osmocom_Types.ttcn
index 0c26f02..484f6d1 100644
--- a/library/Osmocom_Types.ttcn
+++ b/library/Osmocom_Types.ttcn
@@ -1,6 +1,7 @@
 module Osmocom_Types {
 	type integer uint8_t (0..255) with { variant "unsigned 8 bit" };
 	type integer uint16_t (0..65535) with { variant "unsigned 16 bit" };
+	type integer uint24_t (0..16777215) with { variant "unsigned 24 bit" };
 	type integer uint32_t (0..4294967295) with { variant "unsigned 32 bit" };
 
 	type integer int8_t (-128..127) with { variant "8 bit" };