py/parsenum: Use int instead of mp_int_t for parsing float exponent.

There is no need to use the mp_int_t type which may be 64-bits wide, there
is enough bit-width in a normal int to parse reasonable exponents.  Using
int helps to reduce code size for 64-bit ports, especially nan-boxing
builds.  (Similarly for the "dig" variable which is now an unsigned int.)

py/parsenum.c

@@ -227,10 +227,10 @@ mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool
         // string should be a decimal number
         parse_dec_in_t in = PARSE_DEC_IN_INTG;
         bool exp_neg = false;
-        mp_int_t exp_val = 0;
-        mp_int_t exp_extra = 0;
+        int exp_val = 0;
+        int exp_extra = 0;
         while (str < top) {
-            mp_uint_t dig = *str++;
+            unsigned int dig = *str++;
             if ('0' <= dig && dig <= '9') {
                 dig -= '0';
                 if (in == PARSE_DEC_IN_EXP) {