Add swift-futures

[dfunckt]

Pull Request Description

Adds swift-futures: https://github.com/dfunckt/swift-futures

Futures is a relatively large library, makes heavy use of value types and generics (which seem to be stressing the compiler quite a bit) and also includes a C target, which overall I think make for a valuable addition to the source compatibility suite.

Acceptance Criteria

To be accepted into the Swift source compatibility test suite, a project must:

• be an Xcode or swift package manager project
• support building on either Linux or macOS
• target Linux, macOS, or iOS/tvOS/watchOS device
• be contained in a publicly accessible git repository
• maintain a project branch that builds against Swift 4.0 5.0 and passes any unit tests
• have maintainers who will commit to resolve issues in a timely manner
• be compatible with the latest GM/Beta versions of Xcode and swiftpm
• add value not already included in the suite
• be licensed with one of the following permissive licenses:
  • BSD
  • MIT
  • Apache License, version 2.0
  • Eclipse Public License
  • Mozilla Public License (MPL) 1.1
  • MPL 2.0
  • CDDL
• pass ./project_precommit_check script run

Ensure project meets all listed requirements before submitting a pull request.

[clackary]

@swift-ci test

[clackary]

Thanks @dfunckt!