Update the DLEq proof for any amount of generators

The two-generator limit wasn't required nor beneficial. This does 
theoretically optimize FROST, yet not for any current constructions. A 
follow up proof which would optimize current constructions has been 
noted in #38.

Adds explicit no_std support to the core DLEq proof.

Closes #34.

crypto/dleq/src/tests/cross_group/mod.rs

@@ -12,10 +12,9 @@ use dalek_ff_group::{self as dfg, EdwardsPoint};
 use transcript::{Transcript, RecommendedTranscript};
 
 use crate::{
-  Generators,
   cross_group::{
     scalar::mutual_scalar_from_bytes,
-    ClassicLinearDLEq, EfficientLinearDLEq, ConciseLinearDLEq, CompromiseLinearDLEq
+    Generators, ClassicLinearDLEq, EfficientLinearDLEq, ConciseLinearDLEq, CompromiseLinearDLEq
   }
 };