Improve the inner node proving mechanism to be general purpose

After working on https://github.com/celestiaorg/nmt/issues/256, which only supports proving the inner nodes of an ideal tree. We could extend the work to be general purpose and allow proving the inner nodes of any tree with any size.

An attempt to do that was in this https://github.com/celestiaorg/nmt/pull/260 where an approach that uses coordinates is followed. However, that design can change if needed and other ways can be explored.

Start by reading issue #256 and pull request #260 to understand the existing ideal-tree proof and the coordinate-based approach. Explore an agreed design for proving inner nodes in trees of arbitrary size, then verify that the resulting mechanism works beyond ideal trees.

Written by the indexing model from the issue text.