Add code generator for convolutional codes

Add python utility to generate .c code with convolutional
encoder/decoder based on polynomial description of the code. If argument
given it'll be interpreted as intended output directory, otherwise
current working directory is used.

Codes for *CCH, CS2/3 and TCH/AFS are generated. Corresponding manual
implementations are removed from tests. This introduce build-time
dependency on python.

The main work for this patch was generously contributed by Sylvain
Munaut.

Fixes: OS#1629
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index f9a8369..5ed9499 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -56,6 +56,18 @@
 
 gsm0502_calc_paging_group;
 
+gsm0503_xcch;
+gsm0503_cs2;
+gsm0503_cs3;
+gsm0503_tch_afs_12_2;
+gsm0503_tch_afs_10_2;
+gsm0503_tch_afs_7_95;
+gsm0503_tch_afs_7_4;
+gsm0503_tch_afs_6_7;
+gsm0503_tch_afs_5_9;
+gsm0503_tch_afs_5_15;
+gsm0503_tch_afs_4_75;
+
 gsm0808_att_tlvdef;
 gsm0808_bssap_name;
 gsm0808_bssmap_name;