VFS Improvements (#259)

This PR refactors all of the methods on `Vfs` from accepting `&mut self` to
accepting `&self` and keeping data wrapped in a mutex. This builds on previous
changes to make reference count file contents and cleans up the last places
where we're returning borrowed data out of the VFS interface.

Once this change lands, there are two possible directions we can go that I see:
* Conservative: Refactor all remaining `&mut Vfs` handles to `&Vfs`
* Interesting: Embrace ref counting by changing `Vfs` methods to accept `self:
  Arc<Self>`, which makes the `VfsEntry` API no longer need an explicit `Vfs`
  argument for its operations.

* Change VfsFetcher to be immutable with internal locking
* Refactor Vfs::would_be_resident
* Refactor Vfs::read_if_not_exists
* Refactor Vfs::raise_file_removed
* Refactor Vfs::raise_file_changed
* Add Vfs::get_internal as bits of Vfs::get
* Switch Vfs to use internal locking
* Migrate all Vfs methods from &mut self to &self
* Make VfsEntry access Vfs immutably
* Remove outer VFS locking (#260)
* Refactor all snapshot middleware to accept &Vfs instead of &mut Vfs
* Remove outer VFS Mutex across the board

src/snapshot_middleware/dir.rs

@@ -20,7 +20,7 @@ pub struct SnapshotDir;
 impl SnapshotMiddleware for SnapshotDir {
     fn from_vfs<F: VfsFetcher>(
         context: &mut InstanceSnapshotContext,
-        vfs: &mut Vfs<F>,
+        vfs: &Vfs<F>,
         entry: &VfsEntry,
     ) -> SnapshotInstanceResult<'static> {
         if entry.is_file() {