Add data structure for watch descriptor management

For watching whole trees - a change which is upcoming - the watch
descriptor IDs will have to be managed in multiple lists. The
InotifyWatchManager encapsulates the managment logic within a nice API.

Sources/Inotify/InotifyWatchManager.swift

@@ -0,0 +1,18 @@
+struct InotifyWatchManager {
+	private var watchPaths: [CInt: String] = [:]
+	private var activeWatches: Set<CInt> = []
+
+	mutating func add(_ path: String, withId watchDescriptor: CInt) {
+		self.watchPaths[watchDescriptor] = path
+		self.activeWatches.insert(watchDescriptor)
+	}
+
+	mutating func remove(forId watchDescriptor: CInt) {
+		self.watchPaths.removeValue(forKey: watchDescriptor)
+		self.activeWatches.remove(watchDescriptor)
+	}
+
+	func path(forId watchDescriptor: CInt) -> String? {
+		return self.watchPaths[watchDescriptor]
+	}
+}