Various improvements motivated by early real-world use in engine code
under development:
- Make `WithDigest` return `Canonical`, since the return value always
has a name and a digest.
- Introduce a `NamedTagged` type, which can be used for strongly typing
cases where something must have a name and a tag.
- Rename `ParseNamed` to `WithName`, and create a
`ParseNamed` that accepts tags and digests (returning a `Named` type).
The new `ParseNamed` makes code using the reference package much less
verbose, since typical use cases require a name, and this is an
additional type assertion and error case after every `Parse` call.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
16eea0cc47