Adjustments to walk recursion

[gaurav-arya]

Adjusts the walk::AbstractWalk interface to naturally support recursion through walk(x, ys...) and deprecates an old fmap signature; resolves #62.

One subtle point: I wanted to leave the recurse first-arg exactly as is which would have made the diff even smaller. However, for the dispatch to work, we needed to restrict the first arg recurse to be an AbstractWalk when implementing walk subtypes, in which case it made more sense to call it outer_walk, or something of that nature. [This is a breaking change for people who have defined custom walks, since custom subtypes of walk will need to make sure to have this dispatch. This leads to IMO the cleanest API, but I could use something more clunky than walk(x, ys...) for the recursive walk and avoid the breaking change if desired. The main point of this PR is to show that something like the recurse closure is not necessary]

[gaurav-arya]

Changing how custom walks are defined would be really breaking for Optimisers, it seems. Closing and trying again.