Add puzzle 011 (#16)

This puzzle introduces directional portals. It also refines the rules
for portals in general and improves collision logic for beams using
portals.

Additionally, support for sequential mask resolution was added. For
example, this is the case if multiple beams enter portals at the same
time.

src/components/collision.js

@@ -1,27 +1,49 @@
 import { StepState } from './step'
 
 export class Collision {
-  constructor (index, points, items) {
+  constructor (index, points, beam, item) {
+    const items = [beam]
+
+    if (item !== undefined) {
+      items.push(item)
+    }
+
+    this.beam = beam
     this.index = index
 
     // The item that was collided with
-    this.item = items[1]
+    this.item = item
     this.itemIds = items.map((item) => item.id)
     this.items = items
 
     // The first collision point
     this.point = points[0]
     this.points = points
+
+    // Check if the collision is with self
+    this.withSelf = beam.equals(item)
+  }
+
+  copy (settings) {
+    return new Collision(
+      settings.index ?? this.index,
+      settings.points ?? this.points,
+      settings.beam ?? this.beam,
+      settings.item ?? this.item
+    )
   }
 
   equals (other) {
     return other && other.point.equals(this.point) &&
-      other.items.length === this.items.length &&
       other.items.every((item) => this.has(item))
   }
 
-  has (beam) {
-    return this.itemIds.includes(beam.id)
+  has (item) {
+    return this.itemIds.includes(item.id)
+  }
+
+  mirror () {
+    return this.copy({ beam: this.item, item: this.beam })
   }
 }