Add actions for getting 1 or 2 sockets

[chris-martin]

Requiring exactly one or two sockets seems like a really common use case, so I thought we could support it explicitly, with good error messages for when an unexpected number of sockets is present.

I also added a re-export of the Socket type here for convenience.

[ddfisher]

Thanks for the PR! I'll try to find some time this week to take a more detailed look at it, but here are some initial thoughts:

• Overall, convenience methods for getting one or two activated sockets seem like a nice thing to have, so I'd be glad to merge some version of this PR.
• From a quick look, the implementation looks good.
• I'm not in love with the names for the new methods. I like that they're quite specific, but they also feel a bit clunky/verbose. I don't have any concrete alternatives, though.
• I'm not sure I like re-exporting Socket -- it doesn't feel idiomatic to me. Are there other libraries that do similar things?

[chris-martin]

I considered getActivatedSockets1 and getActivatedSockets2, would that sound better?

Some context for why I wanted the Socket re-export: I was just passing the Socket straight into a function from another library, I ended up having to add a network dependency to my project just for the sole purpose of importing Socket so I could write an expression type-signature, which seemed sort of ridiculous. I don't really have a strong opinion, and there doesn't seem to be any popular consensus on the subject, so I'm happy to take it out if you don't want it.

[chris-martin]

Updated:

• Removed the Socket re-export
• Renamed the functions to getActivatedSockets1 and getActivatedSockets2
• Added example usage docs for all three functions