chore(rpc): add convenience from impl (#1686)

crates/rpc/rpc-types/src/eth/index.rs

@@ -1,10 +1,12 @@
+use reth_primitives::U256;
 use serde::{
     de::{Error, Visitor},
     Deserialize, Deserializer, Serialize, Serializer,
 };
 use std::fmt;
 
-/// A hex encoded or decimal index
+/// A hex encoded or decimal index that's intended to be used as a rust index, hence it's
+/// deserialized into a `usize`.
 #[derive(Debug, PartialEq, Eq, Hash, Clone, Copy, Default)]
 pub struct Index(usize);
 
@@ -14,6 +16,12 @@ impl From<Index> for usize {
     }
 }
 
+impl From<Index> for U256 {
+    fn from(idx: Index) -> Self {
+        U256::from(idx.0)
+    }
+}
+
 impl Serialize for Index {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where