Add a basic Queue data structure

Implemented as a linked list.

Terrain2Tests/QueueTests.swift

@@ -0,0 +1,51 @@
+//
+//  QueueTests.swift
+//  Terrain2Tests
+//
+//  Created by Eryn Wells on 11/10/18.
+//  Copyright © 2018 Eryn Wells. All rights reserved.
+//
+
+import XCTest
+@testable import Terrain2
+
+class QueueTests: XCTestCase {
+
+    func testEnqueue() {
+        var queue = Queue<Int>()
+        queue.enqueue(item: 1)
+        queue.enqueue(item: 2)
+        queue.enqueue(item: 3)
+        XCTAssertEqual(queue.count, 3)
+        XCTAssertEqual(queue.head?.item, 1)
+        XCTAssertEqual(queue.head?.next?.item, 2)
+        XCTAssertEqual(queue.head?.next?.next?.item, 3)
+    }
+
+    func testDequeue() {
+        var queue = Queue<Int>()
+        queue.enqueue(item: 1)
+        queue.enqueue(item: 2)
+        queue.enqueue(item: 3)
+
+        XCTAssertEqual(queue.count, 3)
+        XCTAssertEqual(queue.head?.item, 1)
+        XCTAssertEqual(queue.tail?.item, 3)
+
+        XCTAssertEqual(queue.dequeue(), 1)
+        XCTAssertEqual(queue.count, 2)
+        XCTAssertEqual(queue.head?.item, 2)
+        XCTAssertEqual(queue.tail?.item, 3)
+
+        XCTAssertEqual(queue.dequeue(), 2)
+        XCTAssertEqual(queue.count, 1)
+        XCTAssertEqual(queue.head?.item, 3)
+        XCTAssertEqual(queue.tail?.item, 3)
+
+        XCTAssertEqual(queue.dequeue(), 3)
+        XCTAssertEqual(queue.count, 0)
+        XCTAssertNil(queue.head)
+        XCTAssertNil(queue.tail)
+    }
+
+}