There are a few tricky details to call out here. The first is the
`TransactableValueMarker` trait. This is strictly a marker (like
`Sized`, for example) to give some control over what types can be used
as value types in `Entity` instances. This expression is needed due
to the network of `Into` and `From` relations between the parts of
valid `Entity` instances. This allows to drop the `IntoThing`
work-around trait and use the established patterns. (Observe that
`KnownEntid` makes this a little harder, due to the cross-crate
consistency restrictions.)
The second is that we can get rid `{add,retract}_kw`, since the
network of relations expresses the coercions directly.
The third is that this commit doesn't change the name `TermBuilder`,
even though it is now building `Entity` instances. This is because
there's _already_ an `EntityBuilder` which fixes the `EntityPlace`.
It's not clear whether the existing entity building interface should
be removed or whether both should be renamed. That can be follow-up.
With the transition toward parsing with `rust-peg` and away from
`combine`, we're not using some of the many helpers we built to
support our unusual `combine` usage. They can just go!
I elected to keep Tolstoy using `failure::Error`, because Tolstoy
looks rather more like a high-level application (and will continue to
do so for a while) than a production-ready mid- or low-level API.
