Make GridPoint support dictionaries

Add hashValue propertiy and Hashable protocol

Sources/AOC2018/tools.swift

@@ -19,9 +19,14 @@ func == <T:Equatable> (tuple1:(T,T),tuple2:(T,T)) -> Bool
    return (tuple1.0 == tuple2.0) && (tuple1.1 == tuple2.1)
 }
 
-struct GridPoint: Equatable, Comparable {
+struct GridPoint: Equatable, Comparable, Hashable {
     var X = 0
     var Y = 0
+    var hashValue: Int {
+        get {
+            return X.hashValue ^ Y.hashValue
+        }
+    }
 
     static func == (lhs: GridPoint, rhs: GridPoint) -> Bool {
         return (lhs.X == rhs.X) && (lhs.Y == rhs.Y)