bench: Don't depend on FieldElement and elligator_ristretto_flavor

An Elligator benchmark was added for the "lizard" extensions, and it
made sure it was only measuring the Elligator part of the work by
expanding a set of bytes into a FieldElement ahead of time. However,
this requires the FieldElement alias to be pub, and depends on the
elligator_ristretto_flavor helper function being pub as well. Changing
it to test the lizard-based entry point means we're diverging less
from upstream, and it's more realistic anyway.

curve25519-dalek/benches/dalek_benchmarks.rs

@@ -7,7 +7,6 @@ use criterion::{
 };
 
 use curve25519_dalek::constants;
-use curve25519_dalek::field::FieldElement;
 use curve25519_dalek::scalar::Scalar;
 
 static BATCH_SIZES: [usize; 5] = [1, 2, 4, 8, 16];
@@ -245,11 +244,8 @@ mod ristretto_benches {
     }
 
     fn elligator<M: Measurement>(c: &mut BenchmarkGroup<M>) {
-        let fe_bytes = [0u8; 32];
-        let fe = FieldElement::from_bytes(&fe_bytes);
-
         c.bench_function("RistrettoPoint Elligator", |b| {
-            b.iter(|| RistrettoPoint::elligator_ristretto_flavor(&fe));
+            b.iter(|| RistrettoPoint::from_uniform_bytes_single_elligator(&[0u8; 32]));
         });
     }
 

curve25519-dalek/src/ristretto.rs

@@ -653,7 +653,7 @@ impl RistrettoPoint {
     ///
     /// This method is not public because it's just used for hashing
     /// to a point -- proper elligator support is deferred for now.
-    pub fn elligator_ristretto_flavor(r_0: &FieldElement) -> RistrettoPoint {
+    pub(crate) fn elligator_ristretto_flavor(r_0: &FieldElement) -> RistrettoPoint {
         let i = &constants::SQRT_M1;
         let d = &constants::EDWARDS_D;
         let one_minus_d_sq = &constants::ONE_MINUS_EDWARDS_D_SQUARED;