Create a rsmt2d.Tree interface wrapper for NamespacedMerkleTree

[evan-forbes]

Summary

Per the closed issue in nmtIn order to recover block data using rsmt2d.RepairExtendedDataSquare, we need a wrapper around nmt.NamespacedMerkleTree that conforms to rsmt2d.Tree interface and is aware of how we are incorporating namespaces into erasure data.

Problem Definition

Since rsmt2d is not aware of namespaces, this wrapper will also have to provide the appropriate namespace to the underlying NamespaceMerkleTree.

Proposal

If we simply keep track of square size and how many leaves have been pushed, then we can do just that. Use the included namespace for the first half of the tree, and use the hardcoded parity namespace for every leaf in the second half.

type ErasuredNamespacedMerkleTree struct {
	squareSize uint64
	pushCount  uint64
	options    []Option
	tree       *NamespacedMerkleTree
}

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[evan-forbes]

This should also help solve #172