store: support adding existing structures

[hackaugusto]

Describe your changes

fixes #136

implemented the extend trait for the merkle store, consume an iterator over inner nodes. this can be used to add existing structures into the store.

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.

[bobbinth]

Thank you! I left one comment inline. One other thing: I was thinking that we'd remove most with_* and add_* methods after this. Is there a reason to keep them around?

[hackaugusto]

One other thing: I was thinking that we'd remove most with_* and add_* methods after this. Is there a reason to keep them around?

MerklePath and MerklePathSet need it (these don't have the inner node iterator), should I add them?

edit: btw, the merkle path doesn't iterate over the inner nodes (it doesn't have them), it actually needs to compute instead. I have a PR for that already #122 , once we get that it should be easy to transform it to a InnerNodeInfo

edit2: forgot to mention, we can probably remove the methods for the MMR / SMT, let me know if I should remove these.

[bobbinth]

One other thing: I was thinking that we'd remove most with_* and add_* methods after this. Is there a reason to keep them around?

MerklePath and MerklePathSet need it (these don't have the inner node iterator), should I add them?

Let's remove all methods that we can and we'll remove the remaining ones later.