switch to std::experimental::optional

[jfirebaugh]

std::experimental::optional is likely to become a non-experimental part of the standard library in the next revision, and is more complete than our mapbox/variant-based implementation. It's supported by gcc 4.9 and clang 3.5; need to check on iOS/OS X.

For brevity, I recommend we define mbgl/util/optional.hpp:

#include <experimental/optional>

namespace mbgl {
using optional = std::experimental::optional;
}

And then use unqualified optional<T> wherever needed in the mbgl namespace.

[kkaefer]

👍