Add std.typecons.among

Been a couple of days and the discussion has died down a little, so here's an implementation of the `among` utility function, as suggested [in this thread](http://forum.dlang.org/thread/l8nocr$1dv5$1@digitalmars.com) by @andralex.

I've (somewhat arbitrarily) put it in `std.typecons` for now; suggestions on its proper location would be much appreciated.

I still can't think of a solid template constraint for the predicated function that doesn't require the definition of additional auxiliary templates.

Please destroy.