fix: resolve Rust compilation errors by fixing imports and missing types

src/digest_engine/none.rs

@@ -1,7 +1,6 @@
 use anyhow::Result;
 use crate::digest_engine::DigestEngine;
 use std::io::Write;
-use std::io;
 
 #[derive(Debug, Eq, PartialEq, Clone, Default)]
 pub struct DigestEngineNone {}