Swift Package Manager support

[stefanhp]

This is a follow-up to PR #6.

As mentioned in #6 we are using this code in a framework of ours that runs on iOS, watchOS and macOS. We recently updated our build toolchain to support SPM. In order to do this we also implemented SPM on SwiftDigest (this PR).

Changes

• Implement SPM using swift tools version 5.1 (Xcode 11.2.1)
• Add support for Linux and tvOS
• Add Linux in Travis CI build matrix

How to use

In development mode you can reference the package by adding:

dependencies: [.package(url: "[email protected]:Myotest/SwiftDigest.git", .branch("feature/swift-package-manager"))],

in your Package.swift.

Once the PR is merged and tagged with a version, for example 1.0.0 you would use:

dependencies: [.package(url: "https://github.com/NikolaiRuhe/SwiftDigest.git", from: "1.0.0"],