Mark cross_group as experimental

While all of Serai can be argued as experimental, the DLEq proof is 
especially so, as it's lacking any formal proofs over its theory.

Also adds doc(hidden) to the generic DLEqProof, now prefixed with __.

crypto/dleq/src/tests/mod.rs

@@ -1,4 +1,4 @@
-#[cfg(feature = "cross_group")]
+#[cfg(feature = "experimental_cross_group")]
 mod cross_group;
 
 use hex_literal::hex;