Sprout a Lens module

See: #4

As we learn how to Swift; we're stepping away from the SwiftUI/etc.
system and focusing more on building a library for doing the filtering.

We'll start with downloading a file from the Internet, saving it, then
running it through a LensStack, and saving a copy at each stage.

This way, we can start to diagnose if/how the filters are working
without futzing about with SwiftUI; which adds a confounding factor when
trying to figure things out.

lens.swift

@@ -0,0 +1,86 @@
+import Foundation
+import AppKit
+
+// Persists data to the filesystem sans any wrappers.
+// Probably should look into using UserDefaults or FileManager.
+// See: https://programmingwithswift.com/save-images-locally-with-swift-5/
+func save(data:Data) {
+  guard let url = URL(string: "file:///Users/zee/x.png") else {
+    print("Bad URL")
+    return
+  }
+  print("Writing \(url)")
+  try! data.write(to: url)
+  print("Wrote \(url)")
+}
+
+// Retrieves data from a string and passes it to the callback
+func download(urlString: String, callback: @escaping (Data) -> ()) {
+  guard let url = URL(string: urlString) else {
+    print("Bad URL")
+    return
+  }
+  print("A")
+  let task = URLSession.shared.dataTask(with: url) { data, response, error in
+    print("B")
+    guard let data = data, error == nil else {
+      print("OH NO")
+      return
+    }
+    print("C")
+
+    print(data)
+    callback(data)
+    print("D")
+  }
+  // tasks start paused; so we need to start them
+  task.resume()
+  // And because we are in a swift command line program,
+  // we're going to sleep so that the exectuion doesn't stop
+  // before the file is downloaded
+  while !task.progress.isFinished {
+    print(".")
+    sleep(1)
+  }
+}
+
+
+
+
+
+class Filter {
+
+}
+
+// A Fingerprint is a random-number generator that is used by Lenses
+// to adjust the image.
+class Fingerprint {
+
+}
+
+// A Lens applied to a CGImage returns a new CGImage the filter transformations applied
+class Lens {
+  var fingerprint: Fingerprint
+  var filter: Filter
+  init(fingerprint:Fingerprint, filter:Filter) {
+    self.fingerprint = fingerprint
+    self.filter = filter
+  }
+
+  func apply(image: CGImage) -> CGImage {
+    return image
+  }
+}
+
+let aang = "https://vignette.wikia.nocookie.net/avatar/images/3/37/Aang_inhales.png/revision/latest?cb=20130814113013"
+
+download(urlString: aang) { (data) -> () in
+  save(data: data)
+  let image = NSImage(data: data)!
+  // We _think_ we can get a CGImage off of the NSImage, which means
+  // we should be able to run the CGImage through a Lens
+  print(image)
+  let lens = Lens(fingerprint: Fingerprint(), filter: Filter())
+  print(lens)
+}
+