* Add EntityBuilder.add_kw.
This allows you to skip your own attribute lookups, at the cost of
potentially doing the work more than once.
Also does value type checking.
* Add TypedValue::instant(micros).
* Add From<f64> for TypedValue.
* Add lookup_values_for_attribute to Conn.
* Add q_explain to Queryable.
* Expose an iterator over FindSpec's columns.
* Export edn from mentat crate. Export QueryExecutionResult.
* Implement Display for Variable and Element.
* Introduce a `kw` macro.
This allows you to write:
```rust
kw!(:foo/bar)
```
instead of
```rust
NamespacedKeyword::new("foo", "bar")
```
… and it's more efficient, too.
Add `mentat::open`, eliminate use of `mentat_db` in some places.
This includes two other changes:
* Split transact to expose an interface for TermWithTempIds.
* Return TxReport from each InProgress operation, not from commit.
