Since #5882, the three VS 2015 jobs of AppVeyor fail on develop:
- the detection of keys that can be indexed instantiated std::hash for
every string type, which VS 2015 rejects with a static_assert for a type
it does not hash (alt_string): std::hash is now looked at only for
objects in a vector
- the frame stack copied its frames with std::copy to a raw pointer, which
VS 2015 reports as unsafe (C4996, an error with /WX): a loop now
Signed-off-by: Niels Lohmann <mail@nlohmann.me>