add functions for bit-reversal

Sometimes we need stuff like reversing every bit in each byte (but not
the byte-order).
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 2b4ac6e..6c3cb33 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1,5 +1,5 @@
 if ENABLE_TESTS
-SUBDIRS = timer sms ussd smscb
+SUBDIRS = timer sms ussd smscb bits
 if ENABLE_MSGFILE
 SUBDIRS += msgfile
 endif