Update for release 0.9.0

[alexlarsson]

Bump version to 0.9.0

This is the first stable release of composefs. Starting now, we
guarantee a stable library ABI and a binary stable file format. The
later means that any image build from an identical lcfs_node tree and
identical write options, will produce a file that is binary identical
to a later run even with a different version. The same is true for
an mkcomposefs run with the same options.

Major changes since 0.1.4:

* All required overlayfs xattr changes are now upstream and the
  corresponding image generation changes have been made in
  composefs. This includes support for escaping overlayfs xattrs and
  whiteouts for nested overlayfs mounts.

* fs-verity built-in signature support was dropped in favour of
  userspace signatures.

* The erofs iamges now uses the new bloom filter for faster xattr
   lookups. This is backward compatible and old erofs version still
   work (sans the speedup).

* Files can now be inlined in the erofs image to avoid overhead of
  using redirections for small files.

* There is a new API to regenerate a lcfs_node tree from a composefs
  image file.

* There is a new composefs-info tool that lets you dump info about
  images, including what objects it refers to and which ones are
  missing from a given basedir.

* Various fixes, cleanups and new tests

[giuseppe]

LGTM