Prefix generate_mid_from_tmsi with a gsm48_, create a new method
to binary encode the imsi. Add a unit test for parsing and decoding.
The implementation can parse the data it generated and the
last octet seems to be filled with the end mark.
The existing gsm_04_08.c implementation is mixing BSC and MSC
behavior. Move some simple parsing and generation functions over
to gsm_04_08_utils.c to allow a different MSC to define the policy.