benchmark commit step and ark counterparts

this MR adds a benchmark for

• the KZG10 trusted setup creating of ark-poly-commit
• the KZG10 commit of ark-poly-commit
• our own implement of the commit in zk::commit

there is also a slight improvement to the previous benchmarking of our zk::setup: the degree of the trusted setup is now computed once and for all before the benchmarking loop starts, because it's not what is of interest, let's not benchmark it.

benches/setup.rs

@@ -141,7 +141,7 @@ where
                 kzg10::Powers::<E> {
                     powers_of_g: ark_std::borrow::Cow::Owned(powers_of_g),
                     powers_of_gamma_g: ark_std::borrow::Cow::Owned(powers_of_gamma_g),
-                };
+                }
             })
         },
     );