A wee bit of a cleanup of the API

- Move some method over to defaulted methods
 - Use Objects rather than Preconditions

src/main/java/dan200/computercraft/api/filesystem/IMount.java

@@ -90,7 +90,7 @@ public interface IMount
      * @throws IOException If the file does not exist, or could not be opened.
      */
     @Nonnull
-    @SuppressWarnings("deprecation")
+    @SuppressWarnings( "deprecation" )
     default ReadableByteChannel openChannelForRead( @Nonnull String path ) throws IOException
     {
         return Channels.newChannel( openForRead( path ) );