Translate C to Swift

Split the `CMicrophonePitchTracker` C module into new `ZenFFT` and
`ZenPTrack` Swift modules.

When compiling the new modules with `-Ounchecked` performance is within
2% of the C version. Without `-Ounchecked` it has a ~15% overhead.

With `-Ounchecked`:

```console
$ hyperfine --warmup 5 -m 20 ./pitchbench-c ./pitchbench-swift
Benchmark 1: ./pitchbench-c
  Time (mean ± σ):     392.8 ms ±   0.8 ms    [User: 379.5 ms, System: 9.4 ms]
  Range (min … max):   391.8 ms … 394.9 ms    20 runs
 
Benchmark 2: ./pitchbench-swift
  Time (mean ± σ):     401.5 ms ±   0.7 ms    [User: 386.9 ms, System: 10.7 ms]
  Range (min … max):   400.6 ms … 402.8 ms    20 runs
 
Summary
  './pitchbench-c' ran
    1.02 ± 0.00 times faster than './pitchbench-swift'
```

Without `-Ounchecked`:

```console
$ hyperfine --warmup 5 -m 20 ./pitchbench-c ./pitchbench-swift
Benchmark 1: ./pitchbench-c
  Time (mean ± σ):     393.0 ms ±   1.4 ms    [User: 380.3 ms, System: 8.7 ms]
  Range (min … max):   391.9 ms … 398.4 ms    20 runs

Benchmark 2: ./pitchbench-swift
  Time (mean ± σ):     451.1 ms ±   1.0 ms    [User: 436.0 ms, System: 11.0 ms]
  Range (min … max):   449.7 ms … 454.7 ms    20 runs

Summary
  './pitchbench-c' ran
    1.15 ± 0.00 times faster than './pitchbench-swift'
```

Packages/MicrophonePitchDetector/Package.swift

@@ -1,5 +1,4 @@
 // swift-tools-version:5.9
-// The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription
 
@@ -16,10 +15,27 @@ let package = Package(
         )
     ],
     targets: [
-        .executableTarget(name: "pitchbench", dependencies: ["PitchRecording"]),
-        .target(name: "PitchRecording", dependencies: ["MicrophonePitchDetector"]),
-        .target(name: "MicrophonePitchDetector", dependencies: ["CMicrophonePitchDetector"]),
-        .target(name: "CMicrophonePitchDetector"),
+        .executableTarget(
+            name: "pitchbench",
+            dependencies: ["PitchRecording"]
+        ),
+        .target(
+            name: "PitchRecording",
+            dependencies: ["MicrophonePitchDetector"]
+        ),
+        .target(
+            name: "MicrophonePitchDetector",
+            dependencies: ["ZenPTrack"]
+        ),
+        .target(
+            name: "ZenPTrack",
+            dependencies: ["ZenFFT"],
+            swiftSettings: [.unsafeFlags(["-Ounchecked"], .when(configuration: .release))]
+        ),
+        .target(
+            name: "ZenFFT",
+            swiftSettings: [.unsafeFlags(["-Ounchecked"], .when(configuration: .release))]
+        ),
         .testTarget(
             name: "MicrophonePitchDetectorTests",
             dependencies: [