This feature flag is not part of the public API, but only for
conditionally rendering sub-circuit graphs. Renaming it stops the
feature from being enabled in the workspace when we only want to enable
the `dev-graph` feature flag of `halo2_proofs`, which _is_ part of its
public API.
There are two existing patterns for constructing a gate from a set of
constraints with a common selector:
- Create an iterator of constraints, where each constraint includes the
selector:
```
vec![
("foo", selector.clone() * foo),
("bar", selector.clone() * bar),
("baz", selector * bar),
]
```
This requires the user to write O(n) `selector.clone()` calls.
- Create an iterator of constraints, and then map the selector in:
```
vec![
("foo", foo),
("bar", bar),
("baz", bar),
].into_iter().map(move |(name, poly)| (name, selector.clone() * poly))
```
This looks cleaner overall, but the API is not as intuitive, and it
is messier when the constraints are named.
The `Constraints` struct provides a third, clearer API:
```
Constraints::with_selector(
selector,
vec![
("foo", foo),
("bar", bar),
("baz", bar),
],
)
```
This focuses on the structure of the constraints, and handles the
selector application for the user.