add KeychainAccess

[kishikawakatsumi]

Pull Request Description

KeychainAccess is a simple Swift wrapper for Keychain API that works on iOS, watchOS, tvOS and macOS. KeychainAccess framework being added here is licensed under MIT.

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 3.0 compatibility mode
  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 ./check script run

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

[lplarson]

Looks good. Thanks!