WIP: Add static multicasters

[benlesh]

This is another go at adding proposed static multicast functions. Returning ConnectableObservable from operators is/was a mistake.

TODO:

• Settle on names (currently, multicastFrom (static), multicastWith (operator), publishFrom, etc)
• Deprecate publish operator variants that do not have a supplied selector
• Add tests
• Add more docs

NOTE:

Also adds a test showing a weakness of our current approach with regards to composing custom observable types through operator chains. This could probably get added at another time, but I wanted to be sure to capture it while I was thinking of it, and seeing some of the ConnectableObservable code reminded me of the issue.

To discuss:

• Names? xWith makes sense, but what about xFrom?
• Should we have a static variant for every permutation, or just for multicast?
• How frequently do we use these things?

[benlesh]

Related #5431

[cartant]

LGTM. I like the names and the approach. This ought to make the transition easier than it would have been if multicast had been made static without being renamed, etc.

I'll have another look when this is no longer a WIP.

[jgbpercy]

Related #3833

[benlesh]

Closing in favor of #5634