Remove max_depth in native MerkleTree (#442)

This simplifies the MerkleTree (and container) API.
Defer the max depth check when assigning the witness (merkle proof siblings) to the merkle tree circuit.

In this implementation the native Merkle Tree branches grow as much as they needed.  There are no checks of max depth in the merkle tree.  All keys are 256 bits (I added a debug_assert for this); so in the worst case a path will have depth 256.  It can't have a longer depth because the `insert` method calls `prove_nonexistence` which errors if the key already exists; another one may exist which must be different and thus require a path <= 256 depth. 

Resolve #436

src/frontend/custom.rs

@@ -356,7 +356,7 @@ mod tests {
         let mut mp_builder = MainPodBuilder::new(&params, vd_set);
 
         let set_values: HashSet<Value> = [1, 2, 3].iter().map(|i| Value::from(*i)).collect();
-        let s1 = Set::new(params.max_depth_mt_containers, set_values)?;
+        let s1 = Set::new(set_values);
         let s2 = 1;
 
         let set_contains = mp_builder.pub_op(Operation::set_contains(s1, s2))?;