Factor Link and LinkCollection-related code out into its own package.

[mwadams]

I have code where I would like to depend on Menes's representation of links and link collections (via WebLink and ILinkCollection without drawing in the rest of Menes's abstractions.

To that end, I would like to factor out those two types into their own nuget package.

At present, there are no other low-level representations I want to bring across which makes this a bit sparse. So I am in two minds about it.