add noexcept on assignment operators, and a test check those

[lightmare]

You can check how the test ran on master here: https://travis-ci.org/lightmare/variant/builds/106630852

With this patch Travis/AppVeyor will still fail, that's intended.

Except for Xcode6 having some issue with is_nothrow_move_constructible, all other builds on travis will have only 2 failed is_nothrow_destructible checks. Those will go away once ~variant() is declared noexcept (unconditionally), but I didn't wan't to touch that line so that it doesn't conflict with reverting previous commits or any other fix to the destructor issue.

AppVeyor has more than 2 failing is_nothrow_destructible, that might mean it's indeed broken on MSVC and so the checks will need to be skipped/removed.

Note: I removed private functions copy_assign and move_assign -- the names were misleading, they did destroy&construct, not assign -- and copied their bodies to respective operators.

[tomilov]

Note, that in C++11 and later destructors marked by noexcept implicitly (if destructors of all non static members and base classes are noexcept in its turn). There no need to mark them intentionally.

[lightmare]

Shall I rebase this on master? I sooo much hate merge conflicts :spaghetti: