rust: add tink::subtle::constant_time_compare

Based on the `subtle` crate's `ConstantTimeEq` trait.

Author: daviddrysdale

rust/Cargo.lock

@@ -16,6 +16,7 @@
 
 use crate::TinkError;
 use digest::Digest;
+use subtle::ConstantTimeEq;
 
 mod hkdf;
 pub use self::hkdf::*;
@@ -127,3 +128,8 @@ pub fn new_big_int_from_hex(s : &str) (*big.Int, error) {
     return ret, nil
 }
 */
+
+/// Compare two slices in constant time. Return `true` if they are equal, `false` otherwise.
+pub fn constant_time_compare(left: &[u8], right: &[u8]) -> bool {
+    left.ct_eq(right).into()
+}