Repair must return Merkle branches for each non-nil leaf on error

[adlerjohn]

Ref #44 (comment)

[adlerjohn]

Transcluding my comment from Discord:

@liamsi so while looking into #53, I realized different Merkle trees have different prove function signatures:

1. Regular Merkle tree returns a bunch of stuff: https://github.com/lazyledger/merkletree/blob/6901c4c3c75f7ab8a0f11707cb0eb4e477f9074f/tree.go
2. NMT returns a proof struct: https://github.com/lazyledger/nmt/blob/ddcc72040149c115f83b2199eafabf3127ae12ac/nmt.go

You accurately pointed out in a TODO that the Prove method would need some looking into https://github.com/lazyledger/rsmt2d/blob/5a692bff96defabeaf8c928510c1894255e97761/tree.go#L22-L23. I see two alternatives:

1. Make all trees conform to the same Prove method. This might be not possible since they fundamentally return different stuff.
2. Defer constructing proofs to the caller, which should know exactly which tree is used and can handle the specific return values. Note that as-is, there is sufficient data for the caller to construct Merkle proofs (all repaired shares are returned). Also note that within the repair function there may not be enough data for all Merkle proofs to be constructed; some proofs must come from when a share is downloaded. As such, the caller will need to handle attaching the appropriate proofs regardless. I prefer this approach.

With (2) being the preferred approach.