Add codecs for EF_SPN and GSM strings via construct

This will replace the hand-crafted codec for EF_SPN
by a struct definition using the construct library.
Old encoders are updated and kept for API compatibility
but are not used internally anymore.

New data structures:
* Rpad(Adapter): Right-padded bytestring (0xff, adjustable)
* GsmStringAdapter(Adapter): Codec for "SMS default 7-bit
	coded alphabet as defined int TS 23.038" using
	the gsm0338 library.
* GsmString(n): Convenient wrapper of both above

Adjustments:
* utils: update+deprecate old dec_spn(), enc_spn()
* remove refs to deprecated functions

Change-Id: Ia1d3a3835933bac0002b7c52511481dd8094b994

contrib/jenkins.sh

@@ -23,6 +23,7 @@ pip install cmd2
 pip install jsonpath-ng
 pip install construct
 pip install bidict
+pip install gsm0338
 
 # Execute automatically discovered unit tests first
 python -m unittest discover -v -s tests/