Address kibit suggestions.
Fix vault.data and test.
Improve file blob store logic.

Now checks for existing blobs and makes stored blob files read-only.
Refactor vault.blob namespace.

Switched to wrapper functions around the BlobStore protocol which can
default to using the `*blob-store*` dynamic var.
Change the way digest algorithms are handled.

Instead of blob-stores having a specified digest algorithm, all stores
must be agnostic to the identifiers used with them, as long as the
digest algorithm is supported. Instead, the algorithm used defaults to
the value of the `*digest-algorithm*` dynamic var.
Fix errors in vault tool.