Use black_box to help obscure the dalek-ff-group bool -> Choice conversion

I have no idea if this will actually help, yet it can't hurt.

Feature gated due to MSRV requirements.

Fixes #242.

crypto/dalek-ff-group/Cargo.toml

@@ -29,3 +29,6 @@ curve25519-dalek = "^3.2"
 
 [dev-dependencies]
 ff-group-tests = { path = "../ff-group-tests" }
+
+[features]
+black_box = []

crypto/dalek-ff-group/src/lib.rs

@@ -43,7 +43,14 @@ pub mod field;
 
 // Convert a boolean to a Choice in a *presumably* constant time manner
 fn choice(value: bool) -> Choice {
-  Choice::from(u8::from(value))
+  #[cfg(not(feature = "black_box"))]
+  let res = Choice::from(u8::from(value));
+  #[cfg(feature = "black_box")]
+  let res = {
+    use core::hint::black_box;
+    Choice::from(black_box(u8::from(black_box(value))))
+  };
+  res
 }
 
 macro_rules! deref_borrow {