The YAML file names have an optional `:label` portion that becomes the
latter part of the filename. Reconstructed paths (eg. from
`serdes-dependencies`) don't have those labels.

This change makes the YAML ingestion code able to find a file with a
human-readable label even if the request didn't include it.
No ambiguity results because the file names are always based on the
unique serdes `:id`, usually an `entity_id`.

Cam suggested most of these changes in another PR, and I'm tackling them
here.

Serialization of Databases, Tables, Fields

This brought a few core changes:
- Add `serdes-entity-id` to abstract the field used for the ID
- Pass the options to `extract-one` so it can eg. do encryption things.
- Handle dates in YAML storage and ingestion
- `:serdes/meta` now holds the entire hierarchy, not just the leaf model+ID pair.

There's an open problem here about the right way to handle secrets like
a database's password. Do we assume both sides have the same
`MB_ENCRYPTION_SECRET_KEY`? Provide a serdes-specific password the user
just made up, and every secret gets decrypted with the source key, encrypted with
the serdes key, stored, decrypted with the serdes key, and encrypted with
the destination key?