hashes: Strongly type the sha256d::HashEngine

Currently we are using a type alias for the `sha256d::HashEngine`.

Type alias' allow for potential mixing of types, a `sha256d::HashEngine`
struct can better serve our users with not much additional complexity or
maintenance burden.

bitcoin/src/crypto/sighash.rs

@@ -59,12 +59,12 @@ impl_message_from_hash!(SegwitV0Sighash);
 // but outside of it, it should not be possible to construct these hash
 // types from arbitrary data (except by casting via to/from_byte_array).
 impl LegacySighash {
-    fn engine() -> sha256::HashEngine { sha256d::Hash::engine() }
-    fn from_engine(e: sha256::HashEngine) -> Self { Self(sha256d::Hash::from_engine(e)) }
+    fn engine() -> sha256d::HashEngine { sha256d::Hash::engine() }
+    fn from_engine(e: sha256d::HashEngine) -> Self { Self(sha256d::Hash::from_engine(e)) }
 }
 impl SegwitV0Sighash {
-    fn engine() -> sha256::HashEngine { sha256d::Hash::engine() }
-    fn from_engine(e: sha256::HashEngine) -> Self { Self(sha256d::Hash::from_engine(e)) }
+    fn engine() -> sha256d::HashEngine { sha256d::Hash::engine() }
+    fn from_engine(e: sha256d::HashEngine) -> Self { Self(sha256d::Hash::from_engine(e)) }
 }
 
 sha256t_hash_newtype! {

hashes/src/impls.rs

@@ -6,7 +6,7 @@
 
 use bitcoin_io::impl_write;
 
-use crate::{hmac, ripemd160, sha1, sha256, sha512, siphash24, HashEngine};
+use crate::{hmac, ripemd160, sha1, sha256, sha256d, sha512, siphash24, HashEngine};
 
 impl_write!(
     sha1::HashEngine,
@@ -26,6 +26,15 @@ impl_write!(
     |_us| { Ok(()) }
 );
 
+impl_write!(
+    sha256d::HashEngine,
+    |us: &mut sha256d::HashEngine, buf| {
+        us.input(buf);
+        Ok(buf.len())
+    },
+    |_us| { Ok(()) }
+);
+
 impl_write!(
     sha512::HashEngine,
     |us: &mut sha512::HashEngine, buf| {

hashes/src/sha256d.rs

@@ -13,10 +13,30 @@ crate::internal_macros::hash_type! {
     "Output of the SHA256d hash function."
 }
 
-type HashEngine = sha256::HashEngine;
+/// Engine to compute SHA256d hash function.
+#[derive(Clone)]
+pub struct HashEngine(sha256::HashEngine);
 
-fn from_engine(e: sha256::HashEngine) -> Hash {
-    let sha2 = sha256::Hash::from_engine(e);
+impl HashEngine {
+    /// Creates a new SHA256d hash engine.
+    pub const fn new() -> Self { Self(sha256::HashEngine::new()) }
+}
+
+impl Default for HashEngine {
+    fn default() -> Self { Self::new() }
+}
+
+impl crate::HashEngine for HashEngine {
+    type MidState = sha256::Midstate;
+    fn midstate(&self) -> Self::MidState { self.0.midstate() }
+
+    const BLOCK_SIZE: usize = 64; // Same as sha256::HashEngine::BLOCK_SIZE;
+    fn input(&mut self, data: &[u8]) { self.0.input(data) }
+    fn n_bytes_hashed(&self) -> usize { self.0.n_bytes_hashed() }
+}
+
+fn from_engine(e: HashEngine) -> Hash {
+    let sha2 = sha256::Hash::from_engine(e.0);
     let sha2d = sha256::Hash::hash(&sha2[..]);
 
     let mut ret = [0; 32];