Properly providing operators for users was too complicated, and it is
judged as being out of Hana's scope.
Instead, operators are now provided by helper classes in the detail::
namespace. Now, we can be as dirty as we want, since it is not part of
the interface anymore.
Fixes#138Closes#30
This is because using variable templates limits the objects to being
constexpr, or to not being constexpr (but then initializing the object
becomes an issue).
This is also a step towards being compilable by GCC 4.9, but that only
would not justify the change.
