Implement watching a path

Each inotify instance produces events for paths in its watch list. Each
item in the watch list is identified by its watch descriptor. Different
paths can be watched for different events.

Sources/Inotify/Inotify.swift

@@ -2,6 +2,7 @@ import CInotify
 
 public actor Inotify {
 	private let fd: Int32
+	private var watches: [Int32: String] = [:]
 
 	public init() throws {
 		self.fd = inotify_init1(Int32(IN_NONBLOCK | IN_CLOEXEC))
@@ -10,6 +11,16 @@ public actor Inotify {
 		}
 	}
 
+	@discardableResult
+	public func addWatch(path: String, mask: InotifyEventMask) throws -> Int32 {
+		let wd = inotify_add_watch(self.fd, path, mask.rawValue)
+		guard wd >= 0 else {
+			throw InotifyError.addWatchFailed(path: path, errno: cinotify_get_errno())
+		}
+		watches[wd] = path
+		return wd
+	}
+
 	deinit {
 		cinotify_deinit(self.fd)
 	}