Add SPM package manifest for Swift 4.2

Author: tsolomko

Package@swift-4.2.swift

@@ -0,0 +1,17 @@
+// swift-tools-version:4.0
+import PackageDescription
+
+let package = Package(
+    name: "BitByteData",
+    products: [
+        .library(
+            name: "BitByteData",
+            targets: ["BitByteData"])
+    ],
+    targets: [
+        .target(name: "BitByteData", path: "Sources"),
+        .testTarget(name: "BitByteDataTests", dependencies: ["BitByteData"]),
+        .testTarget(name: "BitByteDataBenchmarks", dependencies: ["BitByteData"])
+    ],
+    swiftLanguageVersions: [4]
+)