refactor(storage-proofs): all clippy lint unit_arg

We use the `black_box` function in a bunch of places, at times the
function argument returns unit, this is to be expected since the
argument is not used for its return value.

Instruct clippy to set the `unit_arg` lint to allow.

storage-proofs/core/benches/drgraph.rs

@@ -2,6 +2,7 @@ use criterion::{black_box, criterion_group, criterion_main, Criterion, Parameter
 use storage_proofs_core::drgraph::*;
 use storage_proofs_core::hasher::pedersen::*;
 
+#[allow(clippy::unit_arg)]
 fn drgraph(c: &mut Criterion) {
     let params = vec![12, 24, 128, 1024];
 

storage-proofs/porep/benches/parents.rs

@@ -46,6 +46,7 @@ fn parents_loop<H: Hasher, G: Graph<H>>(graph: &G, parents: &mut [u32]) {
         .collect()
 }
 
+#[allow(clippy::unit_arg)]
 fn parents_loop_benchmark(cc: &mut Criterion) {
     let sizes = vec![10, 50, 1000];