Make util phexdigit array reusable

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>

src/test/uint256_tests.cpp

@@ -1,6 +1,7 @@
 #include <boost/test/unit_test.hpp>
 
 #include "uint256.h"
+#include <string>
 
 BOOST_AUTO_TEST_SUITE(uint256_tests)
 
@@ -15,4 +16,12 @@ BOOST_AUTO_TEST_CASE(uint256_equality)
     BOOST_CHECK(num1+num2 == num3+num2);
 }
 
+BOOST_AUTO_TEST_CASE(uint256_hex)
+{
+    std::string hexStr = "d35583ed493a5eee756931353144f558e6a9ab3ad6024a63ced7f10daf7faad9";
+    uint256 num1;
+    num1.SetHex(hexStr);
+    BOOST_CHECK(num1.GetHex() == hexStr);
+}
+
 BOOST_AUTO_TEST_SUITE_END()