util.bit53: Add left- and right shift operations

While not used by anything in Prosody, it is known to be used by some
3rd party modules.

util/bit53.lua

@@ -3,5 +3,7 @@ return {
 	band   = function (a, b) return a & b end;
 	bor    = function (a, b) return a | b end;
 	bxor   = function (a, b) return a ~ b end;
+	rshift = function (a, n) return a >> n end;
+	lshift = function (a, n) return a << n end;
 };