this patch adds a basic UL HARQ entity for NR.
The patch also updates some interfaces between MAC/RA/HARQ,
i.e. get_temp_crnti().
It also adds a Msg3 unit test.
building on the previous refactor this patch now adds support
for peridoic BSR reporting (using short BSR). It furthermore does
the following changes:
* add BSR packing
* add proc_bsr_nr unit test
* move mac_nr test code into test folder under src (needs to be done with other test code too)
* refactor some common methods to mac_common.cc
* add common mux_base class
* move UL PDU generation to mux class
* add logical channel registration to MAC, MUX, BSR
* add initial proc BSR
* add basic MAC NR test
* rework MAC interfaces