Create module for Szk proof system (#1013)

Szk wraps an FLP proof system and uses an Xof to derive joint random
coins using a method inspired by Prio3. However, unlike Prio3, Szk
is designed to prove validity of an input that is already secret-shared
between two parties. This is necessary for Mastic, where the input
will be shared via a VIDPF instead of linearly.

Co-authored-by: Hannah Davis <hannah.e.davis@seagate.com>

src/flp.rs

@@ -57,6 +57,8 @@ use std::convert::TryFrom;
 use std::fmt::Debug;
 
 pub mod gadgets;
+#[cfg(all(feature = "experimental", test))]
+pub mod szk;
 pub mod types;
 
 /// Errors propagated by methods in this module.