Group revamp (#261)

* Revamp `KeGroup` trait

* Update dependencies

* Fix `hash_to_scalar` using `OprfGroup` instead of `KeGroup`

* Relax constraints on associated types of `KeGroup`

* Improve `KeGroup` implementation on `Curve`

* Improve `KeyExchange` trait

* Fix new Clippy 1.59 warnings

src/util.rs

@@ -0,0 +1,41 @@
+// Copyright (c) Facebook, Inc. and its affiliates.
+//
+// This source code is licensed under both the MIT license found in the
+// LICENSE-MIT file in the root directory of this source tree and the Apache
+// License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+// of this source tree.
+
+//! Utility functions.
+
+#[cfg(test)]
+pub(crate) fn test_zeroize_on_drop<T: Sized>(value: &mut T) {
+    drop_manually(value);
+
+    test_zeroized(value);
+}
+
+#[cfg(test)]
+pub(crate) fn test_zeroized<T: Sized>(value: &mut T) {
+    use std::{mem, slice, vec};
+
+    let test =
+        unsafe { slice::from_raw_parts(value as *const _ as *const u8, mem::size_of::<T>()) };
+
+    assert_eq!(test, vec![0; mem::size_of::<T>()]);
+}
+
+#[cfg(test)]
+pub(crate) fn drop_manually<T: Sized>(value: &mut T) {
+    use std::{mem, ptr, vec};
+
+    assert!(mem::needs_drop::<T>());
+    let mut test_holder = vec![value];
+    let ptr = &mut *test_holder[0] as *mut T;
+
+    unsafe {
+        test_holder.set_len(0);
+        ptr::drop_in_place(ptr);
+    }
+
+    assert_eq!(test_holder.capacity(), 1);
+}