Add Zipper.*_within APIs that operate on subtrees

[zachallaun]

Currently, regardless of the focus of a zipper, functions like Zipper.find/2 will continue to search nodes outside of the focus (depth-first pre-order). There are cases where it's useful to operate only within the bounds of the current focus, however.

For instance:

zipper
|> Zipper.find(...) # find some parent node
|> Zipper.find_within(...) # find some child within the parent node
|> Zipper.update(...) # update the child somehow
|> Zipper.root() # navigate back to the root of the entire AST

There are other functions in Zipper, like traverse, that would also benefit from a *_within variant.

[NickNeck]

That sounds like a good idea, @zachallaun.

The traverse functions already operate on the subtree, so they do not need the *_within version.

Perhaps we could also simply introduce an within/2 function.

zipper
|> Zipper.find(...) # find some parent node
|> Zipper.within(fn z -> Zipper.find(z, ...)) # find some child within the parent node
|> Zipper.update(...) # update the child somehow
|> Zipper.root() # navigate back to the root of the entire AST