Changed xcode version used in CI; changed swift version .swift-version file; changed swift version in podspec; changed swift badge in readme; consistently use double-quotes for strings in podspec; upgrade project to Xcode 9.3 and new project format; use Xcode's "new build system".
Use recommended settings for TestSWCompression; move SWCompression above TestSWCompression in workspace; remove internal workspace from SWCompression; remove platform-specific schemes and targets from SWCompression; enable support for all Darwin platforms in one target/scheme; set deployment targets as minimal as possible; set correct rpath for TestSWCompression; remove unwanted files from TestSWCompression; reorder files in TestSWCompression; add correct test action to SWCompression scheme; remove code-signing.