uint256: Update constructors to c++11, make ONE static

Replace the memset with C++11 value/aggregate initialisation of
the m_data array, which still ensures the unspecified values end
up as zero-initialised.

This then allows changing UINT256_ONE() from dynamically allocating an
object, to a simpler referencing a static allocation.

src/test/uint256_tests.cpp

@@ -278,4 +278,10 @@ BOOST_AUTO_TEST_CASE( operator_with_self )
     BOOST_CHECK(v == UintToArith256(uint256S("0")));
 }
 
+BOOST_AUTO_TEST_CASE( check_ONE )
+{
+    uint256 one = uint256S("0000000000000000000000000000000000000000000000000000000000000001");
+    BOOST_CHECK_EQUAL(one, uint256::ONE);
+}
+
 BOOST_AUTO_TEST_SUITE_END()