Serialize speed limit annotations

[bsudekum]

This adds a new annotation attribute, segmentMaximumSpeedLimits which represents the maximum speed limit for a segment.

/cc @mapbox/navigation-ios @karenzshea

[1ec5]

The saga continues in #367.

[1ec5]

I revived this PR because it already had tests and handled Autobahns correctly, unlike #367.

[1ec5]

To do before merging:

• Merge Add support for speed limits via maxspeed annotations. #367
• Drop support for deploying backwards to iOS 9.x, macOS 10.11.x, tvOS 9.x, and watchOS 2.x: Require iOS 10.0 et al. #379
• Replace SpeedUnit with Measurement<UnitSpeed>: Add support for speed limits via maxspeed annotations. #367 (comment)
• Convert each measurement to the unit of the corresponding step
• Subclass Measurement<UnitSpeed> as SignpostedSpeed, factoring in the indicated sign style: Add support for speed limits via maxspeed annotations. #367 (comment)

[1ec5]

The SPM build is failing with:

/Users/distiller/project/Sources/MapboxDirections/Extensions/Measurement.swift:3:11: error: 'Measurement' is only available on OS X 10.12 or newer
extension Measurement where UnitType: UnitSpeed {
          ^

which is weird, because I just merged in #379, which requires macOS 10.12 and above.

[frederoni]

SPM doesn't depend on Xcode, it has its own place to specify supported platforms which I took the liberty of adding to this PR here.

[1ec5]

• Convert each measurement to the unit of the corresponding step
• Subclass Measurement<UnitSpeed> as SignpostedSpeed, factoring in the indicated sign style: Add support for speed limits via maxspeed annotations. #367 (comment)

Ideally MapboxDirections.swift would be responsible for cleaning up the model so that the speed limit attributes are readily usable in UI. Unfortunately, it’s a challenge to associate attributes with steps. Currently the navigation SDK associates congestion attributes with steps, but that’s memory intensive and a lot of inconvenient code.

[1ec5]

87ee659 introduces a new property, RouteLeg.segmentRangesByStep, that makes it easier to get the elements in the segment* properties that correspond to a given step. That mostly addresses the concerns in #250 (comment). For speed limits, the navigation SDK can now use code like this to efficiently determine the speed limit to display:

let distanceTraveled = legProgress.currentStepProgress.distanceTraveled
let index = legProgress.currentStep.shape!.indexedCoordinateFromStart(distance: distanceTraveled).index
// or: let index = legProgress.currentStep.shape!.closestCoordinate(to: coordinate)!.index
let legProgress = routeProgress.currentLegProgress
let range = legProgress.leg.segmentRangesByStep[legProgress.stepIndex]
let speedUnit = legProgress.currentStep.speedLimitUnit!
let speedLimit = legProgress.leg.segmentMaximumSpeedLimits[range][index].converted(to: speedUnit)
let speedLimitSignStandard = legProgress.currentStep.speedLimitSignStandard