Changes

trau: factor out V.110 frame conversion functions

Implementation of CSD conversion between TRAU frames and CLEARMODE
inside osmo_trau2rtp() and osmo_rtp2trau() contains lower-level
functions for conversion of individual V.110 frames and for alignment
checking.  Factor these functions out into API, as they are useful
for other applications besides TRAU<->RTP conversion:

* Compression and decompression functions that convert between
  standard (but inefficient) CLEARMODE and compressed-CSD RTP format
  of TW-TS-007;

* Implementations of IWFs that terminate CSD RTP streams at the CN
  and support both CLEARMODE and TW-TS-007.

Change-Id: I3f51254b4b985a0f8499c375b61069ca24f77ae8