[mmr] Use iterator for MmrLeaves

[seunlanlege]

When getting the leaves for a subtree, we copy into two new arrays. We can avoid this by using an iterator that sets the offset and length for subtree leaves.

[CJ13th]

Hey @seunlanlege, is this issue still relevant? If it is, I think it could make a "good first issue" for one of the Polkadot Blockchain Academy alumni to work on during morekudos.com/carnival.

[seunlanlege]

Hi yes it's still relevant

[CJ13th]

Great!

I've deployed a Hyperbridge project page morekudos.com/projects/hyperbridge

Going forward, any open issues labelled as good first issue from the repos listed on your project page will automatically appear in the table.

The Kudos Carnival is a 6 week issue solving competition with the aim of getting more PBA alumni contributing to the ecosystem, so feel free to label some existing issues or generate new ones. Issues of all types are being requested, some good categories could be:

• Building example Dapps using your team’s SDK or tech stack
• Improving documentation
• Bug fixing
• Polishing existing features / UI
• New feature requests

[seunlanlege]

Hi @CJ13th can you also add this repo

https://github.com/polytope-labs/sc-simnode

[CJ13th]

@seunlanlege Added!

[Krayt78]

Hello @seunlanlege, i took some time to implement this change, its in this PR #43.
Let me know if any changes are needed or if i didnt understand the issue in the first place xD