Move model helpers to model directory

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

WireGuard/Shared/Model/String+ArrayConversion.swift

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: MIT
+// Copyright © 2018 WireGuard LLC. All Rights Reserved.
+
+import Foundation
+
+extension String {
+
+    func splitToArray(separator: Character = ",", trimmingCharacters: CharacterSet? = nil) -> [String] {
+        return split(separator: separator)
+            .map {
+                if let charSet = trimmingCharacters {
+                    return $0.trimmingCharacters(in: charSet)
+                } else {
+                    return String($0)
+                }
+        }
+    }
+
+}
+
+extension Optional where Wrapped == String {
+
+    func splitToArray(separator: Character = ",", trimmingCharacters: CharacterSet? = nil) -> [String] {
+        switch self {
+        case .none:
+            return []
+        case .some(let wrapped):
+            return wrapped.splitToArray(separator: separator, trimmingCharacters: trimmingCharacters)
+        }
+    }
+
+}