The trick here is to root the resource context within the finalizer. It
only works for mutable objects, or objects with mutable fields, where a
finalizer can be attached by proxy. But that's already pretty good.

Of course immutable struct can't be finalized, but also there's no way
around this; despite initial apperances, _root_resource is semantically
unsound in general. So just bite the bullet and make Blob and BlobTree
mutable.

Uses the new ResourceContexts.detach_context_cleanup to transform
context-based resource handling into finalizer-based resource handling.

Updated to be more explicit the role of the unscoped vs scoped forms of
open.

Use ResourceContexts.jl for resource handling

* Storage driver for data embedded in Data.toml

* Blobs are stored as base64 encoded strings
* Trees are stored as dictionaries

While we're at it, start to clean up storage backend interface.

Also deprecate `abspath(::RelPath)` depending on `pwd()`, as the
assumption that this refers to the filesystem, and that depending on the
global `pwd()` is actually reasonable seem pretty dubious in hindsight.

* Fix for storage backends which don't use ResourceContexts

* Compatibility tests for old backend API