Convert std.random to a package

[Joseph Rushton Wakeling via Digitalmars-d]

Hello all,

As I promised at DConf (though sadly arriving a bit later than 
I'd hoped), I've submitted a PR to convert std.random to a 
package:
https://github.com/D-Programming-Language/phobos/pull/3406

I'd like to ask for some attention to this because, first, I 
think it's important that we agree that this particular package 
structure is correct/logical, and second, I really don't want to 
have to rework this if anyone commits to std.random in the 
meanwhile ;-)

Beyond perhaps documentation issues I hope this PR should be 
fairly non-controversial and easy to approve (or disapprove).

Thanks & best wishes,

      -- Joe