Implement Containers (Dictionary,Set,Array) on top of MerkleTree. And restructure the code. (#55)

* Implement Containers (Dictionary,Set,Array) on top of MerkleTree. And restructure the code.

- Reorganize the code grouping backends, middleware, frontend, (crypto) primitives.
- Add types Dictionary,Set,Array at the middleware layer, so that
  it can be used both by the backend and frontend. The Dictionary, Set,
  Array use the merkletree differently as specified at
f2575d1524/book/src/values.md (dictionary-array-set)
	- The containers introduce the trait Container, which has the
	  method 'cm()'. At the current version this uses a merkletree
	  under the hood, and the method 'cm' returns the merkle root.
- Ideally neither frontend nor backend use the MerkleTree type, and they
  use the wrappers {Dictionary,Set,Array}. Note that the current commit
  the MerkleTree is used at the mock-backend to check internal values, but
  not at the struct types.
- updated the spec's merkletree section updating the defined interface
- add github ci to run the tests

---------

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

book/src/merkletree.md

@@ -145,28 +145,30 @@ For the current use cases, we don't need to prove that the key exists but the va
 
 ```rust
 impl MerkleTree {
-    /// builds a new `MerkleTree` where the leaves contain the given key-values
-    fn new(kvs: HashMap<Value, Value>) -> Self;
-    
     /// returns the root of the tree
-    fn root(&self) -> Result<Hash>;
+    fn root(&self) -> Hash;
+
+    /// returns the value at the given key
+    fn get(&self, key: &Value) -> Result<Value>;
     
+    /// returns a boolean indicating whether the key exists in the tree
+    fn contains(&self, key: &Value) -> bool;
+
     /// returns a proof of existence, which proves that the given key exists in
-    /// the tree. It returns the `value` of the leaf at the given `key`, and
-    /// the `MerkleProof`.
-    fn prove(&self, key: &Value) -> Result<(Value, MerkleProof)>;
-    
+    /// the tree. It returns the `MerkleProof`.
+    fn prove(&self, key: &Value) -> Result<MerkleProof>;
+
     /// returns a proof of non-existence, which proves that the given `key`
     /// does not exist in the tree
     fn prove_nonexistence(&self, key: &Value) -> Result<MerkleProof>;
-    
+
     /// verifies an inclusion proof for the given `key` and `value`
     fn verify(root: Hash, proof: &MerkleProof, key: &Value, value: &Value) -> Result<()>;
-    
+
     /// verifies a non-inclusion proof for the given `key`, that is, the given
     /// `key` does not exist in the tree
     fn verify_nonexistence(root: Hash, proof: &MerkleProof, key: &Value) -> Result<()>;
-    
+
     /// returns an iterator over the leaves of the tree
     fn iter(&self) -> std::collections::hash_map::Iter<Value, Value>;
 }