func DistanceManhattan(p, q gruid.Point) int {
	p = p.Sub(q)
	return abs(p.X) + abs(p.Y)
}
DistanceManhattan computes the taxicab norm (1-norm). See:
https://en.wikipedia.org/wiki/Taxicab_geometry
It can often be used as A* distance heuristic when 4-way movement is used.
package paths
import "github.com/anaseto/gruid"