Remove C::F_len, C::G_len for F_len<C> and G_len<C>

Relies on the ff/group API, instead of the custom Curve type.

Also removes GENERATOR_TABLE, only used by dalek, as we should provide 
our own API for that over ff/group instead. This slows down the FROST 
tests, under debug, by about 0.2-0.3s. Ed25519 and Ristretto together 
take ~2.15 seconds now.

crypto/frost/src/curve/dalek.rs

@@ -11,29 +11,24 @@ macro_rules! dalek_curve {
     $Curve:      ident,
     $Hram:       ident,
     $Point:      ident,
-    $Table:      ident,
 
     $POINT: ident,
-    $TABLE: ident,
 
     $ID:      literal,
     $CONTEXT: literal,
     $chal:    literal,
     $digest:  literal,
   ) => {
-    use dalek_ff_group::{$Point, $Table, $POINT, $TABLE};
+    use dalek_ff_group::{$Point, $POINT};
 
     #[derive(Clone, Copy, PartialEq, Eq, Debug)]
     pub struct $Curve;
     impl Curve for $Curve {
       type F = Scalar;
       type G = $Point;
-      type T = &'static $Table;
 
       const ID: &'static [u8] = $ID;
-
       const GENERATOR: Self::G = $POINT;
-      const GENERATOR_TABLE: Self::T = &$TABLE;
 
       fn random_nonce<R: RngCore + CryptoRng>(secret: Self::F, rng: &mut R) -> Self::F {
         let mut seed = vec![0; 32];
@@ -58,14 +53,6 @@ macro_rules! dalek_curve {
       fn hash_to_F(dst: &[u8], msg: &[u8]) -> Self::F {
         Scalar::from_hash(Sha512::new().chain_update($CONTEXT).chain_update(dst).chain_update(msg))
       }
-
-      fn F_len() -> usize {
-        32
-      }
-
-      fn G_len() -> usize {
-        32
-      }
     }
 
     #[derive(Copy, Clone)]
@@ -84,9 +71,7 @@ dalek_curve!(
   Ristretto,
   IetfRistrettoHram,
   RistrettoPoint,
-  RistrettoBasepointTable,
   RISTRETTO_BASEPOINT_POINT,
-  RISTRETTO_BASEPOINT_TABLE,
   b"ristretto",
   b"FROST-RISTRETTO255-SHA512-v5",
   b"chal",
@@ -98,9 +83,7 @@ dalek_curve!(
   Ed25519,
   IetfEd25519Hram,
   EdwardsPoint,
-  EdwardsBasepointTable,
   ED25519_BASEPOINT_POINT,
-  ED25519_BASEPOINT_TABLE,
   b"edwards25519",
   b"",
   b"",