Release package version 1.4.0

[glessard]

• Include the Sendable conformances (Add Sendable conformances (except FileDescriptor) #115)
• Include Mach.Port (Re-enable Mach.Port #129)
• Bump up toolchain requirement to 5.6, which supports concurrency (Require Swift 5.6 #141)
  • Clean up any conditional compilation branches that are no longer needed.
  • Move the Sendable conformances to the base type declarations.

[glessard]

https://github.com/apple/swift-system/releases/tag/1.4.0