Implement TxGraph::map_anchor method

[evanlinjin]

          Not part of this PR necessarily but the code below seems super complicated and uses changesets for no reason? How about we just have a function on `tx_graph.map_anchor(|anchor| get_new_anchor(anchor))` which consumes self and returns a new `TxGraph` with the new anchor.

Originally posted by @LLFourn in #1084 (comment)

[notmandatory]

Does this still need to be done? @LLFourn @evanlinjin

For now I've put it in the alpha.4 milestone so we don't forget to address it.

[LLFourn]

The code I was complaining about seems to be a bit improved. I think it still might be a nice method to have but we can close this until the need arises.