Add SHA256 midstate conversion to Midstate

diff --git a/hashes/src/sha256/mod.rs b/hashes/src/sha256/mod.rs
index 581ecb48..cf7783fd 100644
--- a/hashes/src/sha256/mod.rs
+++ b/hashes/src/sha256/mod.rs
@@ -95,15 +95,7 @@ impl HashEngine {
     /// Constructs a new [`HashEngine`] from a [`Midstate`].
     ///
     /// Please see docs on [`Midstate`] before using this function.
-    pub fn from_midstate(midstate: Midstate) -> Self {
-        let mut ret = [0; 8];
-        for (ret_val, midstate_bytes) in ret.iter_mut().zip(midstate.as_ref().bitcoin_as_chunks().0)
-        {
-            *ret_val = u32::from_be_bytes(*midstate_bytes);
-        }
-
-        Self { buffer: [0; BLOCK_SIZE], h: ret, bytes_hashed: midstate.bytes_hashed }
-    }
+    pub fn from_midstate(midstate: Midstate) -> Self { midstate.to_engine() }
 
     /// Returns `true` if the midstate can be extracted from this engine.
     ///
@@ -219,6 +211,17 @@ impl Midstate {
     /// Deconstructs the [`Midstate`], returning the underlying byte array and number of bytes hashed.
     pub const fn to_parts(self) -> ([u8; 32], u64) { (self.bytes, self.bytes_hashed) }
 
+    /// Constructs a new [`HashEngine`] from this [`Midstate`].
+    pub fn to_engine(self) -> HashEngine {
+        let mut ret = [0; 8];
+        for (ret_val, midstate_bytes) in ret.iter_mut().zip(self.as_ref().bitcoin_as_chunks().0)
+        {
+            *ret_val = u32::from_be_bytes(*midstate_bytes);
+        }
+
+        HashEngine { buffer: [0; BLOCK_SIZE], h: ret, bytes_hashed: self.bytes_hashed }
+    }
+
     /// Constructs a new midstate for tagged hashes.
     ///
     /// Warning: this function is inefficient. It should be only used in `const` context.
diff --git a/hashes/src/sha256/tests.rs b/hashes/src/sha256/tests.rs
index e73d6dda..e31f3852 100644
--- a/hashes/src/sha256/tests.rs
+++ b/hashes/src/sha256/tests.rs
@@ -132,7 +132,7 @@ fn engine_with_state() {
     ];
 
     let mut engine = sha256::Hash::engine();
-    let midstate_engine = sha256::HashEngine::from_midstate(engine.midstate_unchecked());
+    let midstate_engine = engine.midstate_unchecked().to_engine();
     // Fresh engine and engine initialized with fresh state should have same state
     assert_eq!(engine.h, midstate_engine.h);
 
@@ -147,7 +147,7 @@ fn engine_with_state() {
     let data_vec: &[&[u8]] = &[&[3u8; 1], &[4u8; 63], &[5u8; 65], &[6u8; 66]];
     for data in data_vec {
         let mut engine = engine.clone();
-        let mut midstate_engine = sha256::HashEngine::from_midstate(engine.midstate_unchecked());
+        let mut midstate_engine = engine.midstate_unchecked().to_engine();
         assert_eq!(engine.h, midstate_engine.h);
         assert_eq!(engine.bytes_hashed, midstate_engine.bytes_hashed);
         engine.input(data);
@@ -158,7 +158,7 @@ fn engine_with_state() {
         assert_eq!(hash1, hash2);
     }
 
-    let midstate_engine = sha256::HashEngine::from_midstate(sha256::Midstate::new(MIDSTATE, 64));
+    let midstate_engine = sha256::Midstate::new(MIDSTATE, 64).to_engine();
     let hash = sha256::Hash::from_engine(midstate_engine);
     assert_eq!(hash, sha256::Hash(HASH_EXPECTED));
 }
@@ -227,7 +227,7 @@ fn midstate_error_resume_hashing() {
     let err = engine1.midstate().expect_err("100 bytes not block-aligned");
     assert_eq!(err.unprocessed_bytes().len(), 36);
     // we can resume hashing from err data
-    let mut engine2 = sha256::HashEngine::from_midstate(*err.midstate());
+    let mut engine2 = err.midstate().to_engine();
     engine2.input(err.unprocessed_bytes());
     assert_eq!(sha256::Hash::from_engine(engine1), sha256::Hash::from_engine(engine2));
 }
diff --git a/hashes/src/sha256t/mod.rs b/hashes/src/sha256t/mod.rs
index 39df5a5b..44cf4540 100644
--- a/hashes/src/sha256t/mod.rs
+++ b/hashes/src/sha256t/mod.rs
@@ -133,7 +133,7 @@ pub struct HashEngine<T>(sha256::HashEngine, PhantomData<T>);
 
 impl<T: Tag> Default for HashEngine<T> {
     fn default() -> Self {
-        let tagged = sha256::HashEngine::from_midstate(T::MIDSTATE);
+        let tagged = T::MIDSTATE.to_engine();
         Self(tagged, PhantomData)
     }
 }