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/TSet.swift b/lib/swift/Sources/TSet.swift
index 6891c11..d340fec 100644
--- a/lib/swift/Sources/TSet.swift
+++ b/lib/swift/Sources/TSet.swift
@@ -126,13 +126,8 @@
 
   
   /// Mark: Hashable
-  public var hashValue : Int {
-    let prime = 31
-    var result = 1
-    for element in storage {
-      result = prime &* result &+ element.hashValue
-    }
-    return result
+  public func hash(into hasher: inout Hasher) {
+    hasher.combine(storage)
   }
   
   /// Mark: TSerializable