Refactor zcash_primitives::merkle_tree::CommitmentTreeWitness

- The internal Option wrapper was an unnecessary leftover from when this
  code was directly inside the prover, where Some(x) represents an
  assigned variable.
- CommitmentTreeWitness::from_slice_with_depth is more idiomatic Rust.

zcash_proofs/src/sapling/prover.rs

@@ -117,7 +117,7 @@ impl SaplingProvingContext {
             auth_path: witness
                 .auth_path
                 .iter()
-                .map(|n| n.map(|(node, b)| (node.into(), b)))
+                .map(|(node, b)| Some(((*node).into(), *b)))
                 .collect(),
             anchor: Some(anchor),
         };