Add diesel conversions for "single column types"

[smklein]

The conversions for multi-column types will remain in Omicron, but for single-column types, we define their conversion functions within steno to avoid orphan rule problems.

Tightly coupled with: oxidecomputer/omicron#192

[davepacheco]

If I recall, Steno today doesn't know anything about how the consumer wants to persist state. It just has a notion of what state needs to be persisted when. That felt like a useful decoupling. It seems like this PR adds a bunch of Diesel-specific, SQL-specific knowledge. Do you think that decoupling doesn't make sense or is there not really a great way to do that with Diesel?

[smklein]

If I recall, Steno today doesn't know anything about how the consumer wants to persist state. It just has a notion of what state needs to be persisted when. That felt like a useful decoupling. It seems like this PR adds a bunch of Diesel-specific, SQL-specific knowledge. Do you think that decoupling doesn't make sense or is there not really a great way to do that with Diesel?

Ack. I can make that work - as mentioned in the commit message, we are implementing diesel traits for steno types. Omicron cannot do this, as the trait + structure are both foreign types.

I can accomplish this in Omicron only by using the newtype pattern - though maybe this isn't a big deal for single column types anyway.