This fixes a potential null dereference.

Drop fs.Release() as FileSystem API method.

This should fix GetAttr racing with Release in UnionFs, where the
GetAttr may pickup the size from before the write.

This eases editing files with editors.

Add tests.

Speed-up: around 40% on ls -lR.

Add benchmark.

This should fix Emacs save problems more robustly.

This conveniently puts all timestamps in one int64 variable.

This fixes spurious changes in timestamps.

Make MountState.write private.

Add a test for AutoUnionFs.