c,python: Use little-endina byte order for bigint arithmetics

This better matches what one would expect from the hex or base64
(upcoming) formats.

tests/test_phogen.py

@@ -5,11 +5,11 @@ from passgeny import phogen
 from passgeny import bhash
 
 PHO_GEN_TEST = [
-        [ b"passgeny",  "herang xiasem zitend qibele" ],
-        [ b"phonetic",  "lineum foneum zybale mangur" ],
-        [ b"generator", "latole elitab ackina exprou" ],
-        [ b"password",  "nulize nomere fonici crednt" ],
-        [ b"duck",      "catabb rompor cricin prunsi" ] ]
+        [ b"passgeny",  "vatome iddeda pashis artarc" ],
+        [ b"phonetic",  "joredl qibini kninet ouslis" ],
+        [ b"generator", "vormul zilyth disedl chryso" ],
+        [ b"password",  "wompot ficara uplort lancon" ],
+        [ b"duck",      "inerea iricin ovusia irrisi" ] ]
 
 for pt in PHO_GEN_TEST:
     bh = bhash.Bhash()