Also, add tests to make sure that an empty pair can be EBO'd. This one is very
important because a typical use case is to create a tuple of pairs of empty
types (e.g. in hana::map), and we expect this to be empty.
The commits are db487b2b4 and 76a0ac16a. They were supposed to fix
unit tests failing on Windows due to a missing EBO, but both of these
commits failed to do so. Since I can't test on Windows by myself for
the time being, I'm rolling back these commits and I'll try to fix
this error later.
- Use basic_tuple instead of closure<> for the inner storage
- Make sure the assignment operators are constexpr
- Copy the unit tests from closure<> (closure will eventually be removed anyway)
Fixes#165
