Addition getters for leafs

[Amxx]

The MerkleTree class currently provides mechanism to:

• get hash of a leaf: leafHash(leaf: T): HexString;
• get the index of a leaf: leafLookup(leaf: T): number;
• get an iterator to all leaves (with index): entries(): Iterable<[number, T]>;

What we don't have is:

• a way to get a leaf value given its index.
• a way to get the number of leaves.

Both are directly in tree.values, but it is marked as protected readonly, probably because it also contains the treeIndex which should remain hidden.

I propose we add:

• at(number): T | undefined that gives the value of node at a given position (name?).
• length(): number that returns the number of leaves in the tree (name?).