feat: refactor simple smt to use empty subtree constants

Prior to this commit, there was an internal procedure with the merkle
trees to compute empty sub-tree for arbitrary depths.

However, this isn't ideal as this code can be reused in any merkle
implementation that uses RPO as backend.

This commit introduces a structure that will generate these empty
subtrees values.

src/hash/rpo/digest.rs

@@ -11,7 +11,7 @@ use core::{cmp::Ordering, ops::Deref};
 pub struct RpoDigest([Felt; DIGEST_SIZE]);
 
 impl RpoDigest {
-    pub fn new(value: [Felt; DIGEST_SIZE]) -> Self {
+    pub const fn new(value: [Felt; DIGEST_SIZE]) -> Self {
         Self(value)
     }