This is based on the new nodefs API, and tests the new API for writing
a full-fledged r/w filesystem.

Together with DefaultPermissions, this lets you cd into a nodefs
powered file system.

Remove memtree.

Add an example for constructing an in-memory file system

* Fix deadlock with OnAdd

* return EROFS for missing Create

* Fill Mode using Inode.setEntryOut in all namespace operations

utility classes for symlinks and regular files without backing store.

The node can always call into the file handle if necessary, but the
reverse limits options for implementors.

Instead, define InodeEmbedder, an interface that can only be satisfied
by a struct that embeds Inode.

This is the prevalent style for FUSE libraries in Go.

Thanks to Ryan Lamore (ryanlamore) for spotting this

FOPEN_CACHE_DIR was added in Linux 4.19:

	https://git.kernel.org/linus/6433b8998a

Provide InodeLink/InodeEmbed which implements linking machinery.