Simpler read/write conversions?

We currently have a mechanism for converting between read/write types, modelled after `TryFrom`/`TryInto`, but it is likely that this mechanism could be improved:

- it does not seem to be implemented for records
- It should be possible to implement `From/Into` directly in some cases?
- It would also be nice if this trait had an associated type instead of a generic param, e.g. if it looked like,

```rust
ToOwnedTable {
type Owned;
fn to_owned_table(&self) -> Self::Owned;
}
```

because then you would not need to name the target type. I believe I tried this originally, and one of the issues was having the types declared in separate crates, and running into problems with the orphan rules?

Start by locating the existing read/write conversion traits and their implementations, then check how records and types in separate crates are handled. Compare the current generic-parameter design with associated types and direct From/Into implementations, including orphan-rule constraints; done requires an agreed design and documented scope.

Written by the indexing model from the issue text.