Remove or scope down FileSystemHandle.moveTo()

[pwnall]

FileSystemHandle.moveTo() is a new capability when it can carry out the move atomically. If the source and the target are in different file systems, the implementation needs to fall back to copying FileSystemHandle.copyTo() and removing (FileSystemHandle.remove() / FileSystemDirectoryHandle.removeRecursively()).

In #52 I argue that we shouldn't provide copyTo(). I think the argument applies here as well. I think we should either nix moveTo() for now, or scope it down to the new capability. In the latter case, we'd have the method fail if the move cannot be performed atomically.

[pwnall]

I think this was fixed in #50