py: Fix check of small-int overflow when parsing ints.

Also unifies use of SMALL_INT_FITS macro across parser and runtime.
diff --git a/py/smallint.c b/py/smallint.c
index 186c9c7..5543f12 100644
--- a/py/smallint.c
+++ b/py/smallint.c
@@ -28,6 +28,7 @@
 #include "mpconfig.h"
 #include "qstr.h"
 #include "obj.h"
+#include "smallint.h"
 
 bool mp_small_int_mul_overflow(machine_int_t x, machine_int_t y) {
     // Check for multiply overflow; see CERT INT32-C