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/merkle/mod.rs

@@ -1,12 +1,16 @@
 use super::{
     hash::rpo::{Rpo256, RpoDigest},
     utils::collections::{vec, BTreeMap, Vec},
-    Felt, StarkField, Word, ZERO,
+    Felt, StarkField, Word, WORD_SIZE, ZERO,
 };
 use core::fmt;
 
 // REEXPORTS
 // ================================================================================================
+
+mod empty_roots;
+pub use empty_roots::EmptySubtreeRoots;
+
 mod index;
 pub use index::NodeIndex;