book: Fix book tests by adding missing definitions to SHA-256 page

book/src/design/gadgets/sha256.md

@@ -18,6 +18,15 @@ gadget to consume input in 32-bit chunks.
 The SHA-256 gadget requires a chip with the following instructions:
 
 ```rust
+# extern crate halo2;
+# use halo2::plonk::Error;
+# use std::fmt;
+#
+# trait Chip: Sized {}
+# trait Layouter<C: Chip> {}
+const BLOCK_SIZE: usize = 16;
+const DIGEST_SIZE: usize = 8;
+
 pub trait Sha256Instructions: Chip {
     /// Variable representing the SHA-256 internal state.
     type State: Clone + fmt::Debug;