Common structure of base libraries

Nobody has claimed this yet.

Right now `mapbox-base` is a set of small submodules. That makes hard to support consistency between each small repo. Maybe we should place all these files in one repo? Like a std library.

As example, in VisionSDK we want to use `optional` and `variant` from `mapbox-base` and have a troubles:
- `optional` class is placed inside `std::experimental` namespace, but variant places in `mapbox::util` one.
- `optional` package have different structure from `variant`. `variant` have `include/mapbox` folder that allow me to add include folder to `include_directories` and use `#include "mapbox/variant.hpp"`. But i can't do the same with `optional`. Because `optional.hpp` placed directly inside `optional` submodule directory

Start by reviewing the mapbox-base submodules and comparing the package layouts and namespaces of optional and variant, especially their include paths. Define a common repository structure and consistent public include and namespace conventions, then confirm that the VisionSDK use cases can consume both libraries consistently.

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