Haversine distance should use spherical Earth radius

Nobody has claimed this yet.

This library currently defines the radius of the Earth as 6 373 000 m for the purpose of converting between meters and radians in the Haversine formula:

https://github.com/mapbox/turf-swift/blob/40c7d772c46837aa9e5ceb60d22d8ad97a4be708/Turf/Turf.swift#L7

Apparently this value came from [an old version of turf-distance](https://github.com/Turfjs/turf-distance/blob/cea4c6cbec582bff8880193a3c2cda38ae731967/index.js#L78) that I originally ported to an internal Swift application, before it got copied into another internal Swift application, then copied into the navigation SDK, then moved here. It’s close to the value of 6 372 797.560 856 that [Osmium](https://github.com/Project-OSRM/osrm-backend/blob/bb1f4a025a3cefd3598a38b9d3e55485d1080ec5/third_party/libosmium/include/osmium/geom/haversine.hpp#L58-L59) describes as “Earth’s quadratic mean radius for WGS84”.

These days, Turf.js uses [a spherical approximation](https://github.com/Turfjs/turf/blob/a94151418cb969868fdb42955a19a133512da0fd/packages/turf-helpers/index.js#L4) of 6 671 008.8 m for its radian-to-meter conversion and Haversine formula. The Haversine formula was always meant to be used with a spherical meters-per-radian value. We should probably change this value to match Turf.js. 🙀

/ref Turfjs/turf#978 Turfjs/turf#1012 Turfjs/turf#1176 https://github.com/mapbox/turf-swift/pull/21#discussion_r157246151
/cc @frederoni @bsudekum @captainbarbosa

Start with Turf/Turf.swift at the Earth-radius definition linked in the issue, then compare the Haversine conversion with the referenced Turf.js spherical value. Done means updating the radius to the agreed spherical value and confirming the affected meter-to-radian and Haversine calculations use it consistently.

Written by the indexing model from the issue text.