A blob store on a model file system (Roux.Test.ModelFS), for the
Concuerror scenarios of test/concurrency/blob_test.ex: the store's
directories made, the calling process entered as model OS process "1".
Summary
Functions
The digest of the empty bytes: the entry every empty relation links.
A scratch directory of the calling process's own, made.
A model file system holding an empty store, and the store.