Extract Socket constants in separate file.

[straight-shoota]

Socket::Address and Socket::Addrinfo conceptually don't dependent on
Socket. The shared constants and lib includes are extracted to common.cr to separate concerns more clearly.

[ysbaddaden]

I dont understand. If they're common constants, that they'll be included anyway, and they're still named Socket::X then I expect them to be defined in socket.cr not buried in some other file.

Can we avoid stylistic/opinion changes, and concentrate on fixing actual issues in Socket?

[straight-shoota]

This is not about style, it is about separation of concerns. I'd like socket.cr to focus on the Socket type, not some other stuff that is in its namespace but not solely required by Socket.
The idea is, if you only need Socket::Address, you can just include socket/address instead of the entire package.
While this selective requiring doesn't have any relevant performance impact as it might have in other languages, the expression of explicit dependencies is still valuable.

This PR is in preparation for refactoring the entire socket API. In the process I found it was much easier to separate individual pieces of code into separate files instead of having the main type + some common stuff in the main file.

This PR is not an integral part of the refactoring, that's why I published it separately. But it makes refactoring easier.

[jhass]

So, where do we want to go with this? 18 months later it didn't seem to have been a pain point for anybody given the activity here...

[straight-shoota]

Closing stale PR.