Make the cross-group DLEqProof prove constant time

Instead of having if statements for the bits, it now has constant time 
ops. While there are still if statements guiding the proof itself, they 
aren't dependent on the data within.

crypto/dleq/Cargo.toml

@@ -10,6 +10,8 @@ edition = "2021"
 thiserror = "1"
 rand_core = "0.6"
 
+subtle = "2.4"
+
 transcript = { package = "flexible-transcript", path = "../transcript", version = "0.1" }
 
 ff = "0.12"