parse: Note that fact that parser's small ints are different than VM small int.

Specifically, VM's small ints are 31 bit, while parser's only 28. There's already
MP_OBJ_FITS_SMALL_INT(), so, for clarity, rename MP_FIT_SMALL_INT() to
MP_PARSE_FITS_SMALL_INT().
diff --git a/py/compile.c b/py/compile.c
index c895b26..ef01304 100644
--- a/py/compile.c
+++ b/py/compile.c
@@ -117,7 +117,7 @@
                         assert(0);
                         res = 0;
                     }
-                    if (MP_FIT_SMALL_INT(res)) {
+                    if (MP_PARSE_FITS_SMALL_INT(res)) {
                         pn = mp_parse_node_new_leaf(MP_PARSE_NODE_SMALL_INT, res);
                     }
                 }