Make InterfaceProxy public

This should fix the cross-assembly visibility issues in #98

Also added EditorBrowsableState.Never, and some loud warnings in the docs, to make it clear that this
shouldn't actually be used, even though it is now public.

This all seems to work for me locally, but I can't get it to properly build me a nupkg that will install in my other projects, almost certainly because I'm doing something wrong, but I'm also quite short on time to look into this this week; if you could test that this patch definitely solves #98 when installed in another project that'd be very helpful.

Alternatively you can wait a few days, and I should have time to have a proper go at sorting it out early next week.