[RFC] std.experimental.concepts

[Shammah Chancellor via Digitalmars-d]

I put up a PR for phobos awhile ago for concepts as a library to 
kind of start the discussion around concepts.   There seemed to 
be some interest around the PR, so I have rebased it and fixed 
the formatting.

Please take a look: 
https://github.com/D-Programming-Language/phobos/pull/2627

-Shammah