Use Swift Package Manager to build the project

Xcode is no longer a requirement. This allows Linux/Windows users to build the project, too.
An Xcode project can still be generated running `swift package generate-xcodeproj`.

Author: SimplyDanny

.gitignore

@@ -24,3 +24,7 @@ bin/
 *~
 *.dat
 *.dep
+
+# Swift Package Manager
+.build/
+.swiftpm/

Package.swift

@@ -0,0 +1,15 @@
+// swift-tools-version:5.3
+
+import PackageDescription
+
+let projectName = "bitsyspec"
+
+let package = Package(
+    name: projectName,
+    targets: [
+        .target(
+            name: projectName,
+            path: projectName
+        ),
+    ]
+)

README.md

@@ -25,24 +25,22 @@ implementation, even enabling a test driven approach if desired.
 
 ## Requirements
 
-This version of BitsySpec has been tested with:
+This version of BitsySpec has been tested with
 
- * OS X/macOS 10.14 (Mojave) or later
- * Xcode 11.3
- * Swift 5.0
+ * macOS 12.3 (Monterey) and Ubuntu 20.04 LTS
+ * Xcode 13.3
+ * Swift 5.6
 
-Linux support is currently limited by
-[Swift Foundation](https://github.com/apple/swift-corelibs-foundation) but
-should come eventually.
+although basically only a Swift toolchain is required to build the project. Thus,
+it may also run perfectly fine on Windows.
 
 Swift is an implementation detail of BitsySpec. Your compiler or interpreter
-can be in virtually any language.
+can be implemented in virtually any language.
 
 ## Installation
 
-To 'install' BitsySpec, simply clone and build the repository. You must have
-[Xcode](https://itunes.apple.com/us/app/xcode/id497799835?mt=12)
-and the `xcodebuild` utility installed.
+To 'install' BitsySpec, simply clone and build the repository. You must have the
+[Swift compiler](https://swift.org/download/#releases) available in your `PATH`:
 
 ```bash
 git clone https://github.com/apbendi/bitsyspec.git