encoding: pass RFN to write_immediate_assignment(_reject)()

Those function don't really require the full FN, hence let's pass only
the required information.
This makes the implementation here less dependent on how/if we are able to calculate
full FNs based on RFN: We get an RFN, and we have to encode so that the RFN can be
derived again, so feels less cumbersome having to go through RFN->FN->RFN which may
only cause possible issues if there's some FN timing bug.

3GPP TS 44.018 10.5.2.30 Request Reference:
"The purpose of the Request Reference information element is to provide the random
access information used in the channel request and the frame number, FN modulo 42432"

3GPP TS 44.018 10.5.2.38 Starting Time:
"The purpose of the Starting Time information element is to provide the start TDMA
frame number, FN modulo 42432."

Change-Id: If9b758434c00f2a3868534d5be84946809c989a9
---
M src/bts.cpp
M src/encoding.cpp
M src/encoding.h
M tests/types/TypesTest.cpp
4 files changed, 55 insertions(+), 31 deletions(-)