Make the Fetcher object expose its internal key

[julienw]

Currently, it's possible to specify a key with useFetcher. But how about exposing the key (even the generated one) as a property of the fetcher? This would make it possible to instanciate a fetcher from a parent, and pass the key to children so that they can use it as well. Currently we have to either pass the full fetcher instance, or use an explicit key.

What do you think?