Swift 5.1 support

Use stdlib Result instead of TAsyncResult

Fix: deprecations for `withUnsafeBytes`

Bump CI job Xcode version

Hash (into)

Co-authored-by: Sophie Lambrakis <SLambrakis@users.noreply.github.com>
Co-authored-by: Alexander Edge <alex@alexedge.co.uk>
diff --git a/lib/swift/Sources/TEnum.swift b/lib/swift/Sources/TEnum.swift
index fedfdb1..2e91f63 100644
--- a/lib/swift/Sources/TEnum.swift
+++ b/lib/swift/Sources/TEnum.swift
@@ -24,7 +24,10 @@
 
 extension TEnum {
   public static var thriftType: TType { return .i32 }
-  public var hashValue: Int { return rawValue.hashValue }
+
+  public func hash(into hasher: inout Hasher) {
+    hasher.combine(rawValue)
+  }
 
   public func write(to proto: TProtocol) throws {
     try proto.write(rawValue)