Adds equivalents of turf-line-chunk

[nighthawk]

Also:

• GeoJSON.LineString.length
• GeoJSON.Polygon.LinearRing.circumference

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TODOs:

• Add tests, see https://github.com/Turfjs/turf-line-chunk/tree/master/test
• Review doc comments, pointing to http://turfjs.org/docs/#lineChunk and https://github.com/Turfjs/turf-line-chunk
• Address spanning anti-meridian

[nighthawk]

When this is done, can also contribute this to the upstream repo to resolve mapbox/turf-swift#183

[nighthawk]

Oh, turf-line-chunk does something different than what I implemented here. Rather than adding more points to the existing line, it splits it up into separate features. Hmm, that's not what I need in my use of this, but someone might need it. Could add a new method as GeoJSON.Feature.chunk(length:) which does that and keep the existing methods that work on line strings and linear rings.