extmod/utime_mphal: ticks_diff(): switch arg order, return signed value.

Based on the earlier discussed RFC. Practice showed that the most natural
order for arguments corresponds to mathematical subtraction:

ticks_diff(x, y) <=> x - y

Also, practice showed that in real life, it's hard to order events by time
of occurance a priori, events tend to miss deadlines, etc. and the expected
order breaks. And then there's a need to detect such cases. And ticks_diff
can be used exactly for this purpose, if it returns a signed, instead of
unsigned, value. E.g. if x is scheduled time for event, and y is the current
time, then if ticks_diff(x, y) < 0 then event has missed a deadline (and e.g.
needs to executed ASAP or skipped). Returning in this case a large unsigned
number (like ticks_diff behaved previously) doesn't make sense, and such
"large unsigned number" can't be reliably detected per our definition of
ticks_* function (we don't expose to user level maximum value, it can be
anything, relatively small or relatively large).

extmod/utime_mphal.c

@@ -78,11 +78,11 @@ STATIC mp_obj_t time_ticks_cpu(void) {
 }
 MP_DEFINE_CONST_FUN_OBJ_0(mp_utime_ticks_cpu_obj, time_ticks_cpu);
 
-STATIC mp_obj_t time_ticks_diff(mp_obj_t start_in, mp_obj_t end_in) {
+STATIC mp_obj_t time_ticks_diff(mp_obj_t end_in, mp_obj_t start_in) {
     // we assume that the arguments come from ticks_xx so are small ints
     uint32_t start = MP_OBJ_SMALL_INT_VALUE(start_in);
     uint32_t end = MP_OBJ_SMALL_INT_VALUE(end_in);
-    return MP_OBJ_NEW_SMALL_INT((end - start) & MP_SMALL_INT_POSITIVE_MASK);
+    return MP_OBJ_NEW_SMALL_INT((int32_t)(end - start));
 }
 MP_DEFINE_CONST_FUN_OBJ_2(mp_utime_ticks_diff_obj, time_ticks_diff);