implement the specified sparse merkletree (#82)

* wip

* prototype custom predicates 1b

* feat: implement custom pred recursion

* files reorg, add github CI for rustfmt checks

* start sparsemerkletree. impl add_leaf method, initial Leaf & Intermediate types with methods

* mt: add hash computation of all the nodes in the tree, add method to print the tree to visualize it as a graphviz

* mt: add  (till the leaf) method which is used by get,contains,prove methods

* mt: add verify (of inclusion) method

* mt: update 'down' method to reuse siblings, update get,contains,prove methods (the three use 'down' under the hood)

* Add nonexistence proofs and iterator

* Add iterator test

* migrate usage of old merkletree to the new merkletree impl in POD2 code

---------

Co-authored-by: Eduard S. <eduardsanou@posteo.net>
Co-authored-by: Ahmad <root@ahmadafuni.com>

src/lib.rs

@@ -1,4 +1,5 @@
 pub mod backends;
+pub mod constants;
 pub mod frontend;
 pub mod middleware;
 pub mod primitives;