[Prelim patch] Speed up ints parsing

[Paolo Carlini]

... and the results for sync_with_stdio(false); are even more
impressing (the num_get component is more relevant in this
case):


3.3.2
-----
12.590u 0.040s 0:13.04 96.8%    0+0k 0+0io 215pf+0w


3.4
---
9.220u 0.000s 0:09.36 98.5%     0+0k 0+0io 204pf+0w


3.4 + patch
-----------
1.780u 0.030s 0:01.89 95.7%     0+0k 0+0io 200pf+0w



Paolo.