Directory Restructure (#1163)

* Move tests -> testing

* Directory restructure

* Update Cargo.toml during restructure

* Update Makefile during restructure

* Fix arbitrary path

crypto/eth2_key_derivation/src/secret_bytes.rs

@@ -0,0 +1,29 @@
+use zeroize::Zeroize;
+
+/// Provides a wrapper around a `Vec<u8>` that implements `Zeroize` on `Drop`.
+#[derive(Zeroize)]
+#[zeroize(drop)]
+pub struct SecretBytes(Vec<u8>);
+
+impl SecretBytes {
+    /// Instantiates `Self` with an all-zeros byte array of length `len`.
+    pub fn zero(len: usize) -> Self {
+        Self(vec![0; len])
+    }
+
+    /// Returns a reference to the underlying bytes.
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.0
+    }
+
+    /// Returns a mutable reference to the underlying bytes.
+    pub fn as_mut_bytes(&mut self) -> &mut [u8] {
+        &mut self.0
+    }
+}
+
+impl From<Vec<u8>> for SecretBytes {
+    fn from(vec: Vec<u8>) -> Self {
+        Self(vec)
+    }
+}