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
diff --git a/setup.py b/setup.py
index 08608dc..2ef498f 100644
--- a/setup.py
+++ b/setup.py
@@ -16,6 +16,7 @@
         "jsonpath-ng",
         "construct >= 2.9",
         "bidict",
+        "gsm0338",
     ],
     scripts=[
         'pySim-prog.py',