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/tests/mod.rs

@@ -14,6 +14,7 @@ use crate::{
 // Test suites for public usage
 pub mod curve;
 pub mod schnorr;
+pub mod promote;
 pub mod vectors;
 
 // Literal test definitions to run during `cargo test`