Implement `share` operator

[benlesh]

This is an alias for publish().refCount(), but is honestly preferable for simplicity's sake. It's easier to teach people share() than publish().refCount(), because to a newbie, the latter won't make much sense.

Ideally, this will be implemented by pulling in the publish method via import, rather than depending on it being on the Observable. Since this is a modular library, there is no guarantee publish has been added.

[benlesh]

Also, this one would be a core operator.