feat: Prefetch and cache direction tiles

[jkwening]

Issue Number:

#151

Issue Description:

Add code logic for prefetching tiles at zoom level 18 for points for
polyline via waypoints from origin to destination.

Summary of solution:

1. Add module for calculating x and y coords for given latLng
2. Expand caching logic to handle prefetching directions points of interest

Can this issue be closed?

No - first pass at caching logic

Should any new issues be added as a result of this solution?

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Thanks for contributing!