Consider Generics for Tree

[liamsi]

The tree interface here expects bytes on Push:

rsmt2d/tree.go

Lines 18 to 22 in 80d231f

	// Tree wraps Merkle tree implementations to work with rsmt2d
	type Tree interface {
	Push(data []byte)
	Root() []byte
	}

This makes sense but at the same time we'd rather want to use some explicit type in NMT instead of just aliasing byte slices (see: celestiaorg/nmt#71).

Push could leverage generics potentially instead.

This is a low-priority consideration.

[liamsi]

Looking back at this issue there is no clear benefit over simply using an interface for data instead (just needs a way to serialize from whatever is pushed to the tree).
Closing this issue for now. It is too focused around a specific implementation detail (generics).