fix: implement Write trait for digest engines

src/digest_engine.rs

@@ -37,6 +37,25 @@ pub trait DigestEngine {
     fn update(&mut self, data: &[u8]);
 }
 
+// Dummy writer that implements Write for the none digest engine
+struct DummyWriter;
+
+impl DummyWriter {
+    fn new() -> Self {
+        DummyWriter
+    }
+}
+
+impl Write for DummyWriter {
+    fn write(&mut self, _buf: &[u8]) -> io::Result<usize> {
+        Ok(0)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
 lazy_static! {
     pub static ref DIGEST_PROGRAMS: EnumMap<DigestType, Option<DigestEngineProgram>> = enum_map! {
         DigestType::Sha256 => None,