V0.2.0

Do not wrap `Entropy_unix` in a functor as it is meant to be used directly.  This also lets it be tested directly from utop.

This will also need a patch to `mirage` before 2.0.0 is cut, but I think it makes more sense to just generate a reference to `Entropy_unix.connect` than to jump through `module Entropy = Entropy_unix.Make (OS.Time)`.  Since this is a Unix user, there's no other sensible provider of `V1_LWT.TIME`.

Thoughts, @samoht @djs55 @pqwy?