Right now, we write code like
```rust
match q_once(q, inputs)?.into_tuple()? {
Some(vs) => match (vs.len(), vs.get(0), vs.get(1)) {
(2, &Some(Binding::Scalar(TypedValue::Long(a))), &Some(Binding::Scalar(TypedValue::Instant(ref b)))) => Some((a, b.clone())),
_ => panic!(),
},
None => None,
}
```
to length-check tuples coming out of the database. It can also lead
to a lot of cloning because references are the easiest thing to hand.
This commit allows to write code like
```rust
match q_once(q, inputs)?.into_tuple()? {
Some((Binding::Scalar(TypedValue::Long(a)), Binding::Scalar(TypedValue::Instant(b)))) => Some((a, b)),
Some(_) => panic!(),
None => None,
}
```
which is generally much easier to reason about.
* Refactor AttributeCache populator code for use from pull.
* Pre: add to_value_rc to Cloned.
* Pre: add From<StructuredMap> for Binding.
* Pre: clarify Store::open_empty.
* Pre: StructuredMap cleanup.
* Pre: clean up a doc test.
* Split projector crate. Pass schema to projector.
* CLI support for printing bindings.
* Add and use ConjoiningClauses::derive_types_from_find_spec.
* Define pull types.
* Implement pull on top of the attribute cache layer.
* Add pull support to the projector.
* Parse pull expressions.
* Add simple pull support to connection objects.
* Tests for pull.
* Compile with Rust 1.25.
The only choice involved in this commit is that of replacing the
anonymous lifetime '_ with a named lifetime for the cache; since we're
accepting a Known, which includes the cache in question, I think it's
clear that we expect the function to apply to any given cache
lifetime.
* Review comments.
* Bail on unnamed attribute.
* Make assert_parse_failure_contains safe to use.
* Rework query parser to report better errors for pull.
* Test for mixed wildcard and simple attribute.
