Replace MapboxImageAPI with MapboxStatic.swift

[1ec5]

There’s a lot of overlap between MapboxImageAPI and MapboxStatic.swift. At the same time, each codebase contains distinct functionality: MapboxImageAPI contains tile cover logic, while MapboxStatic.swift also supports overlays and other Static API options. It would be great if this library could depend on MapboxStatic.swift for most of the Static API functionality while continuing to implement the tile cover algorithm.

/cc @avi-c

[jim-martin]

This seems like the best way to address outstanding crashes:

#30, #33, #34