Add an UnbalancedMerkleTree primitive

The reasoning for it is documented with itself. The plan is to use it within
our header for committing to the DAG (allowing one header per epoch, yet
logarithmic proofs for any header within the epoch), the transactions
commitment (allowing logarithmic proofs of a transaction within a block,
without padding), and the events commitment (allowing logarithmic proofs of
unique events within a block, despite events not having a unique ID inherent).

This also defines transaction hashes and performs the necessary modifications
for transactions to be unique.

substrate/primitives/src/lib.rs

@@ -43,6 +43,9 @@ pub mod signals;
 /// Instruction types.
 pub mod instructions;
 
+/// Merkle trees.
+pub mod merkle;
+
 /// The type used to identify block numbers.
 ///
 /// A block's number is its zero-indexed position on the list of blocks which form a blockchain.