Changes

libosmotrau: implement RAA' encoder function

RAA' is the rate adaption function specific to 14.4 kbit/s CSD
services.  It is defined in 3GPP TS 48.020 chapter 11, and consists
of transforming each user data frame of 290 bits (as it would appear
at the radio channel coder interface) into a special A-TRAU frame.
64 kbit/s streams consisting of these A-TRAU frames are meant to
traverse the call switching network from one mobile call leg to
the other, or from a mobile call leg to an IWF.

RAA' transform involves rather bewildering manipulation of payload
bit content, done to prevent any possibility of false A-TRAU frame
alignment in octet-oriented (no frame boundaries) 64 kbit/s transport.
The complexity is such that unit tests are essential, especially
unit tests that cross-check our software implementation against
those made by others.

Fortunately this RAA' function is included in traditional T1/E1 TRAUs,
those that support CSD in 14.4 kbit/s mode and implement it according
to the specs.  Nokia TCSM2 in Themyscira lab includes the necessary
support, and the present patch includes a unit test that verifies
perfect agreement between our software implementation of RAA' encoder
and the historical hw implementation in the old Nokia TRAU.

Related: OS#6167
Change-Id: I11fc1529f5be88fa778c7e05cb11eef58a389d40