Add impl to converto from U256 to [u8; 32]

uint/src/uint.rs

@@ -1369,4 +1369,12 @@ macro_rules! impl_quickcheck_arbitrary_for_uint {
 }
 
 construct_uint!(U256, 4);
-construct_uint!(U512, 8);
+construct_uint!(U512, 8);
+
+impl From<U256> for [u8; 32] {
+	fn from(number: U256) -> Self {
+		let mut arr = [0u8; 32];
+		number.to_big_endian(&mut arr);
+		arr
+	}
+}