class template base_uint had its own private lookup table.
This is saving 256 bytes per instantiation.
The result is not spectacular as bitcoin-qt has only shrinked of
about 1Kb but it is still valid improvement.
Also, I have replaced a for loop with a memset() call.
Made CBigNum::SetHex() use the new HexDigit() function.
Signed-off-by: Olivier Langlois <olivier@olivierlanglois.net>
