Implement Async Batch verification API for groth16

This PR is the first step in getting a groth16 proving system fully
integrated with the rest of zebra. This PR implements the initial async
API, but none of the actual batching logic necessary for our eventual
verifier design.

Once the batch verification API from bellman has been implemented we
will need to swap out the "Batch" type defined in this crate with the
new `batch::Verifier` defined in bellman.

tower-batch/src/service.rs

@@ -7,14 +7,16 @@ use super::{
 
 use crate::semaphore::Semaphore;
 use futures_core::ready;
-use std::task::{Context, Poll};
+use std::{
+    fmt,
+    task::{Context, Poll},
+};
 use tokio::sync::{mpsc, oneshot};
 use tower::Service;
 
 /// Allows batch processing of requests.
 ///
 /// See the module documentation for more details.
-#[derive(Debug)]
 pub struct Batch<T, Request>
 where
     T: Service<BatchControl<Request>>,
@@ -35,6 +37,20 @@ where
     handle: Handle,
 }
 
+impl<T, Request> fmt::Debug for Batch<T, Request>
+where
+    T: Service<BatchControl<Request>>,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let name = std::any::type_name::<Self>();
+        f.debug_struct(name)
+            .field("tx", &self.tx)
+            .field("semaphore", &self.semaphore)
+            .field("handle", &self.handle)
+            .finish()
+    }
+}
+
 impl<T, Request> Batch<T, Request>
 where
     T: Service<BatchControl<Request>>,

zebra-consensus/Cargo.toml

@@ -13,6 +13,8 @@ once_cell = "1.5"
 rand = "0.7"
 redjubjub = "0.2"
 serde = { version = "1", features = ["serde_derive"] }
+bellman = "0.8"
+bls12_381 = "0.3.1"
 
 futures = "0.3.12"
 futures-util = "0.3.6"
@@ -28,6 +30,7 @@ tower-batch = { path = "../tower-batch/" }
 zebra-chain = { path = "../zebra-chain" }
 zebra-state = { path = "../zebra-state" }
 zebra-script = { path = "../zebra-script" }
+pairing = "0.18.0"
 
 [dev-dependencies]
 color-eyre = "0.5.10"

zebra-consensus/src/primitives.rs

@@ -5,5 +5,13 @@ pub mod redjubjub;
 
 /// The maximum batch size for any of the batch verifiers.
 const MAX_BATCH_SIZE: usize = 64;
+
 /// The maximum latency bound for any of the batch verifiers.
 const MAX_BATCH_LATENCY: std::time::Duration = std::time::Duration::from_millis(100);
+
+/// The size of the buffer in the broadcast channels used by batch verifiers.
+///
+/// This bound limits the number of concurrent batches for each verifier.
+/// If tasks delay checking for verifier results, and the bound is too small,
+/// new batches will be rejected with `RecvError`s.
+const BROADCAST_BUFFER_SIZE: usize = 512;