Move second map out of PoseMap

`PoseMap` is intended to be a SAM, and the `map()` overload is only
there for convenience. Making it an extension method cleans up the
interface. If performance is an issue, it can safely be moved back
into the interface.

core/src/main/kotlin/com/acmerobotics/roadrunner/Builders.kt

@@ -546,9 +546,10 @@ class PathBuilder private constructor(
 
 fun interface PoseMap {
     fun map(pose: Pose2dDual<Arclength>): Pose2dDual<Arclength>
-    fun map(pose: Pose2d) = map(Pose2dDual.constant(pose, 1)).value()
 }
 
+fun PoseMap.map(pose: Pose2d) = map(Pose2dDual.constant(pose, 1)).value()
+
 class IdentityPoseMap : PoseMap {
     override fun map(pose: Pose2dDual<Arclength>) = pose
 }