Implemented NioFile

* Implementation of NioFile methods
* Extracted Readable/WritableNioFile into separate classes
** Created SharedFileChannel to allow Readable/WritableNioFile for the
same NioFile to use a single, shared FileChannel
* Added tests for NioFile
* Tests for Readable/WritableNioFile pending

main/filesystem-api/src/main/java/org/cryptomator/filesystem/Mover.java

@@ -11,6 +11,9 @@ package org.cryptomator.filesystem;
 class Mover {
 
 	public static void move(File source, File destination) {
+		if (source == destination) {
+			return;
+		}
 		try (OpenFiles openFiles = DeadlockSafeFileOpener.withWritable(source).andWritable(destination).open()) {
 			openFiles.writable(source).moveTo(openFiles.writable(destination));
 		}

main/filesystem-api/src/main/java/org/cryptomator/filesystem/WritableFile.java

@@ -13,6 +13,13 @@ import java.time.Instant;
 
 public interface WritableFile extends WritableByteChannel {
 
+	/**
+	 * <p>
+	 * Moves this file including content to another.
+	 * <p>
+	 * Moving a file causes itself and the target to be
+	 * {@link WritableFile#close() closed}.
+	 */
 	void moveTo(WritableFile other) throws UncheckedIOException;
 
 	void setLastModified(Instant instant) throws UncheckedIOException;