arith_uint256: remove initialization from byte vector

Remove initialization from vector (as this is only used in the tests).

Also implement SetHex and GetHex in terms of uint256, to avoid
duplicate code as well as avoid endianness issues (as they
work in term of bytes).

src/arith_uint256.h

@@ -57,7 +57,6 @@ public:
     }
 
     explicit base_uint(const std::string& str);
-    explicit base_uint(const std::vector<unsigned char>& vch);
 
     bool operator!() const
     {
@@ -257,7 +256,6 @@ public:
     arith_uint256(const base_uint<256>& b) : base_uint<256>(b) {}
     arith_uint256(uint64_t b) : base_uint<256>(b) {}
     explicit arith_uint256(const std::string& str) : base_uint<256>(str) {}
-    explicit arith_uint256(const std::vector<unsigned char>& vch) : base_uint<256>(vch) {}
 
     /**
      * The "compact" format is a representation of a whole