Add support for Ristretto

Replaces P-256 as the curve used for testing FROST.

crypto/frost/src/tests/literal/mod.rs

@@ -1,3 +1,4 @@
+#[cfg(any(test, feature = "dalek"))]
+mod dalek;
+#[cfg(feature = "kp256")]
 mod kp256;
-#[cfg(feature = "ed25519")]
-mod ed25519;