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/TSocketTransport.swift b/lib/swift/Sources/TSocketTransport.swift
index 2132503..57f6215 100644
--- a/lib/swift/Sources/TSocketTransport.swift
+++ b/lib/swift/Sources/TSocketTransport.swift
@@ -198,7 +198,7 @@
     var writeBuffer = data
     while bytesToWrite > 0 {
       let written = writeBuffer.withUnsafeBytes {
-        Sys.write(socketDescriptor, $0, writeBuffer.count)
+        Sys.write(socketDescriptor, $0.baseAddress!, writeBuffer.count)
       }
       writeBuffer = writeBuffer.subdata(in: written ..< writeBuffer.count)
       bytesToWrite -= written