## Changes
The pattern `errors.Is(err, fs.ErrNotExist)` is common to check for an
error type.
Errors can implement `Is(error) bool` with a custom equivalence checker.
## Tests
New asserts all pass in the integration test.
Adds a DBFS implementation of the `filer.Filer` interface.
The integration tests are reused between the workspace filesystem and
DBFS implementations to ensure identical behavior.
Pieter Noordhuis