Add Merkle tree to Symbolic

[vlasin]

Implement Merkle tree type in Symbolic, borrowing ideas from the already implemented List and Hash data types.

• Define MerkleTree d x type to represent the tree of depth d (i.e., with $2^d$ leafs)
• Define MerkleTreePath d x type (a wrapper around Vector d (Bool (Context x)) or similar) to represent the path to a particular leaf in a tree.
• Define the following functions, filling in the appropriate type constraints:

-- | Finds an element satisfying the constraint
find :: (x -> Bool (Context x)) -> MerkleTree d x -> Maybe (Context x) x

-- | Finds a path to an element satisfying the constraint
findPath :: (x -> Bool (Context x)) -> MerkleTree d x -> Maybe (Context x) (MerkleTreePath d x)

-- | Returns the element corresponding to a path
lookup :: MerkleTree d x -> MerkleTreePath d x -> x

-- | Inserts an element at a specified position in a tree
insert :: MerkleTree d x -> MerkleTreePath d x -> x -> MerkleTree d x

-- | Replaces an element satisfying the constraint. A composition of `findPath` and `insert`
replace :: (x -> Bool (Context x)) ->  MerkleTree d x -> x -> MerkleTree d x

-- | Returns the next path in a tree
incrementPath :: MerkleTreePath d x -> MerkleTreePath d x

• Add tests to verify (among other things) the circuit efficiency of those operations and their compositions. For example, a composition of findPath and lookup should only do d hashes in-circuit (instead of naive 2*d) since we verify the same path from the leaf to the root.