Walk: add breadth-first option

[schomatis]

Currently Walk() only has a WalkDepth() implementation. I would like to add a WalkBreadth() one. It seems straight-forward enough:

• Add an option in walkOptions (and process it in Walk() instead of its WalkDepth() callee).
• Implement a basic (sequential) BFS leveraging the same GetLinks() and visit callbacks.

Before taking a stab at it I need confirmation that this is the right path. Particularly, I see that a BFS search is already implemented in the traverse package which overlaps substantially with the logic in Walk().

[schomatis]

I have a first draft in https://github.com/ipfs/go-merkledag/compare/schomatis/walk/breadth-first?expand=1 but now I'm told the best option would be to drop the Walk API of this repo altogether and go with https://pkg.go.dev/github.com/ipld/go-ipld-prime/traversal#WalkMatching (see #62).

I don't have a clear example of how to use the proposed alternative and since porting to it would demand an unknown amount of time/effort I'm pushing ahead with my implementation here, but this is clearly wrong as I'm duplicating functionality already available.