Replace hex decoding closure with something safer and hopefully more performant

Sources/apk/Utility/DataExtensions.swift

@@ -0,0 +1,32 @@
+/*
+ * darwin-apk © 2024 Gay Pizza Specifications
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import Foundation
+
+extension Data {
+  init?(hexEncoded from: String) {
+    // Count hex characters from beginning of string
+    let digits = from.count(where: \.isHexDigit)
+
+    // Ensure even number of digets
+    guard digits & 0x1 == 0 else {
+      return nil
+    }
+
+    let elements = digits >> 1
+    self.init(capacity: elements)
+
+    // Convert digits
+    var idx = from.startIndex
+    for _ in 0..<elements {
+      let hi = from[idx].hexDigitValue!
+      idx = from.index(after: idx)
+      let lo = from[idx].hexDigitValue!
+      idx = from.index(after: idx)
+      let byte = UInt8(truncatingIfNeeded: lo + hi << 4)
+      self.append(byte)
+    }
+  }
+}