maybe remove "string containing slash-separated names" APIs

[zooko]

Consider going from two ways to do it to one way to do it, if there is an API e.g. NewDirectoryNode?.get_child_at_path() which takes either a list of strings where each string is a name or else a string containing forward-slashes, where each slash-separated thing is a name. The former is more general, and if the lack of the latter isn't too inconvenient for people then let's remove explicit support for it.

Just the one interface might be convenient enough because Python programmers can always take their string-containing-slashes and run ".split('/')" on it before passing it to the Tahoe object.

[warner]

I'm cool with that. The get_child_at_path is a valuable convenience, but it could accept a list of strings and still be just as useful.