Add ability to use fuse-mt without the path translation

[not-jan]

Hi! First of all: thanks for writing this awesome library.

I'm writing a filesystem at the moment and wanted to make use of fuse-mt but the concept of tracking paths doesn't really make sense for my usescase so I've added the ability to instead receive inodes from fuse-mt. This should solve e.g. #21

There is a breaking changed I could not avoid: impl FilesystemMT for Foo has to become impl FilesystemMT<'_> for Foo now. Which is why I've also decided to bump the minor version.

I've also bumped the fuser version to 0.14 and added a new example for the new type of "unmanaged" FilesystemMT.

I've designed most other changes to not break any APIs further which has resulted in RawFileAttr implementing Deref and DerefMut with FileAttr as a target.