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_bhash.py

@@ -8,8 +8,7 @@ bh = passgeny.bhash.Bhash()
 
 bh.from_bytes(TEST_STRING)
 
-# Scan the string in reverse order (::-1) as bhash should use big endian arithmetics
-for c in TEST_STRING[::-1]:
+for c in TEST_STRING:
     hc = bh.modulo(256)
     if hc != c:
         raise Exception("Returned value is {}, expected {}".format(chr(hc), c))