Add helper functions for AMR codec

* add functions to encode/decode various codec paramters from RTP payload with
  AMR frame according to RFC 4867
* those functions are extended version based on code from osmo-bts'
  amr.c by Andreas Eversberg
* add corresponding enum types and strings for logging
* add regression tests

It's useful both to replace manual parsing in osmo-bts with fuctions
covered by test suite and as a debugging helpers for issues related to
AMR.

Change-Id: Ia217679a07d3fbc970f435e20f6eac33d34bd597
Related: OS#1562
Reviewed-on: https://gerrit.osmocom.org/118
Tested-by: Jenkins Builder
Reviewed-by: Holger Freyther <holger@freyther.de>
diff --git a/.gitignore b/.gitignore
index 03ce379..70e2016 100644
--- a/.gitignore
+++ b/.gitignore
@@ -80,6 +80,7 @@
 tests/ringbuf/ringbuf_test
 tests/strrb/strrb_test
 tests/vty/vty_test
+tests/codec/codec_test
 tests/gb/gprs_bssgp_test
 tests/smscb/gsm0341_test
 tests/bitvec/bitvec_test