Simpler read/write conversions?

[cmyr]

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,

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?