Add FROST key promotion

Closes https://github.com/serai-dex/serai/issues/72.

Adds a trait, with a commented impl for a semi-unsafe niche feature, 
which will be used in https://github.com/serai-dex/serai/issues/73.

crypto/frost/src/lib.rs

@@ -14,7 +14,10 @@ mod schnorr;
 
 pub mod curve;
 use curve::Curve;
+
 pub mod key_gen;
+pub mod promote;
+
 pub mod algorithm;
 pub mod sign;