Add first party support for k256 and p256 under feature flags

Given the lack of vectors for k256, it's currently a match of the p256 
spec (with a distinct context string), yet p256 is still always used 
when testing.

crypto/frost/src/lib.rs

@@ -13,6 +13,8 @@ mod schnorr;
 pub mod key_gen;
 pub mod algorithm;
 pub mod sign;
+#[cfg(any(test, feature = "p256", feature = "k256"))]
+pub mod curves;
 
 pub mod tests;