[merkledb] Create a Basic Memory Trie Wrapper

[patrick-ogrady]

Make it easy to generate roots from arbitrary keys and to prove keys exist in said roots.

@dboehm-avalabs mentioned this functionality already exists...we just need to wrap it up nicely for our use.

[dboehm-avalabs]

It isn't perfect, but could flesh out this code a bit more to provide that https://github.com/ava-labs/avalanchego/blob/master/x/merkledb/proof.go#L857

[patrick-ogrady]

Proof verification: https://github.com/ava-labs/avalanchego/blob/18324b4a3b4a2262af33af7ae50620ae17eccbe2/x/merkledb/proof.go#L139

[github-actions]

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[patrick-ogrady]

Blocks: #950