feat(providers): add multicall batch layer (#2174)

* wip

* wip

* wip

* a

* fix draining, more tests, docs

* feat: get_balance

* chore: outdated todo

* test: unasync the closure

* docs

* use json

Author: DaniPopes

crates/provider/src/builder.rs

@@ -3,6 +3,7 @@ use crate::{
         CachedNonceManager, ChainIdFiller, FillerControlFlow, GasFiller, JoinFill, NonceFiller,
         NonceManager, RecommendedFillers, SimpleNonceManager, TxFiller, WalletFiller,
     },
+    layers::{CallBatchLayer, ChainLayer},
     provider::SendableTx,
     Provider, RootProvider,
 };
@@ -266,12 +267,8 @@ impl<L, F, N> ProviderBuilder<L, F, N> {
     /// the client's poll interval based on the average block time for this chain.
     ///
     /// Does nothing to the client with a local transport.
-    pub fn with_chain(
-        self,
-        chain: NamedChain,
-    ) -> ProviderBuilder<Stack<crate::layers::ChainLayer, L>, F, N> {
-        let chain_layer = crate::layers::ChainLayer::from(chain);
-        self.layer(chain_layer)
+    pub fn with_chain(self, chain: NamedChain) -> ProviderBuilder<Stack<ChainLayer, L>, F, N> {
+        self.layer(ChainLayer::new(chain))
     }
 
     /// Finish the layer stack by providing a root [`Provider`], outputting
@@ -398,6 +395,13 @@ impl<L, F, N> ProviderBuilder<L, F, N> {
         let client = ClientBuilder::default().hyper_http(url);
         self.on_client(client)
     }
+
+    /// Aggregate multiple `eth_call` requests into a single batch request using Multicall3.
+    ///
+    /// See [`CallBatchLayer`] for more information.
+    pub fn with_call_batching(self) -> ProviderBuilder<Stack<CallBatchLayer, L>, F, N> {
+        self.layer(CallBatchLayer::new())
+    }
 }
 
 impl<L, F, N: Network> ProviderBuilder<L, F, N> {