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/TApplicationError.swift b/lib/swift/Sources/TApplicationError.swift
index bc39396..cfaf285 100644
--- a/lib/swift/Sources/TApplicationError.swift
+++ b/lib/swift/Sources/TApplicationError.swift
@@ -146,9 +146,12 @@
     try proto.writeFieldStop()
     try proto.writeStructEnd()
   }
-  
-  public var hashValue: Int {
-    return error.thriftErrorCode &+ (message?.hashValue ?? 0)
+}
+
+extension TApplicationError: Hashable {
+  public func hash(into hasher: inout Hasher) {
+    hasher.combine(error.thriftErrorCode)
+    hasher.combine(message)
   }
 }