Expand scope to include additions intended for future Scala versions?

[SethTisue]

@lrytz writes (#317 (comment)):

Should we consider extending the scope of this module to scala-library-future where we can also add new classes for 2.13 / 3.0?

@julienrf and myself agree this is a reasonable idea

@lrytz cautions:

We'd have to make it clear that everything added to the 2.13 release here would eventually end up in the standard library, so we'd have to be just as conservative.

assuming we go ahead with this, we'd want to put something in the readme about it, and also update scala/scala-dev#661

[SethTisue]

as discussed over at #331, new stuff should be in a separate module, this repo will stay backports-only. scala-library-future is a good name IMO

[SethTisue]

or maybe scala-library-next. I'll make the repo soon

[SethTisue]

https://github.com/scala/scala-library-next

[SethTisue]

the scala-library-next repo is now open for business