consensus: Add a script verifier stub

Part of #428.

zebra-consensus/src/verify.rs

@@ -18,6 +18,7 @@ use tower::{buffer::Buffer, Service};
 
 use zebra_chain::block::Block;
 
+mod script;
 mod transaction;
 
 /// Block verification service.

zebra-consensus/src/verify/script.rs

@@ -0,0 +1,22 @@
+//! Script verification for Zebra.
+//!
+//! Verification occurs in multiple stages:
+//!   - getting transactions from blocks or the mempool (disk- or network-bound)
+//!   - context-free verification of scripts, signatures, and proofs (CPU-bound)
+//!   - context-dependent verification of transactions against the chain state
+//!     (awaits an up-to-date chain)
+//!
+//! Verification is provided via a `tower::Service`, to support backpressure and batch
+//! verification.
+//!
+//! This is an internal module. Use `verify::BlockVerifier` for blocks and their
+//! transactions, or `mempool::MempoolTransactionVerifier` for mempool transactions.
+
+/// Internal script verification service.
+///
+/// After verification, the script future completes. State changes are handled by
+/// `BlockVerifier` or `MempoolTransactionVerifier`.
+///
+/// `ScriptVerifier` is not yet implemented.
+#[derive(Default)]
+pub(crate) struct ScriptVerifier {}