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/uint256.cpp

@@ -80,7 +80,4 @@ template std::string base_blob<256>::ToString() const;
 template void base_blob<256>::SetHex(const char*);
 template void base_blob<256>::SetHex(const std::string&);
 
-uint256& UINT256_ONE() {
-    static uint256* one = new uint256(uint256S("0000000000000000000000000000000000000000000000000000000000000001"));
-    return *one;
-}
+const uint256 uint256::ONE(1);