Add Version()

[lczyk]

Exposed the 'version' call from ping as a method on Client

[lczyk]

@bradfitz bump, although you do seem like a busy person.. 🤷‍♀️

[dormando]

This seems kinda nice but does need to be cleaned up. @MarcinKonowalczyk - this PR isn't super ancient so if you're up for addressing the review comment please do so. Else we'll look into it.

[lczyk]

ty for the poke! addressed and rebased on top of master. let me know what you think.

ive changed the functionality to VersionAll and Version(key string) and added docs (but if you think it ought to be only one function, i'm more than happy to change it back.

also something to specifically point out: i think the behaviour of VersionAll ought to be to return empty strings for unconnected instances. the rationale being that just because one instance is not connected ought not to stop one learning about other instances. having said that, there is an argument to be made for a simpler api: just one Version function which just checks all instances and errors out if any is not connected. just thoughts though. again, happy to change it to anything you'd like. 👍

[lczyk]

oh, also i;ve made a tiny clarification to doc of Each and added a static type assertion which makes the code a bit more readable. can split it into a separate PR but i thought it minor enough to stick here.