Moves to SDK style project files

[ChrSteinert]

First step for .NET Standard Support.

I left a few things I believe are not directly necessary like ProjectGuid or BindingRedirects.
Please review and advise!

[panesofglass]

Probably similar to #77 and #80. You could look at #80 for guidance, as I think @enricosada had this working before but then closed his PR. @jackfoxy is also working on this, so you two should collaborate.

[jackfoxy]

@ChrSteinert good work. I made several "improvements" locally. Including adding netstandard2.0 as a target and updating to the latest FSharp.Core.

The travis CI failing may be as simple as adding dotnet : 2.1.4 to .travis.yml, but I am not experienced with Travis.

My "enhanced" local build fails on several (but not all) experimental tests,

Could not load file or assembly 'nunit.framework ...

despite binding redirects that look valid.

The binding redirects are a mess, and should not be necessary in a modern SDK library project. Indeed IMO the entire app.config files should go away. The only way I know for sure to accomplish this is to migrate all the tests to Expecto. That's a bit of a chore.

Perhaps @forki would be so kind as to create a 2.0 branch here, and we could collaborate on that branch.

[ChrSteinert]

Cool!
Is this somewhere so we could have a look and proceed from that state further?

[jackfoxy]

@ChrSteinert see #77 (comment)

[ChrSteinert]

Closing this as we are working on this over here: https://github.com/jackfoxy/FSharpx.Collections/tree/netstandard

[jackfoxy]

Actually the place to work on this is now here https://github.com/fsprojects/FSharpx.Collections/tree/netstandard