### What it does
Warns on any exported `enum`s that are not tagged `#[non_exhaustive]`

### Why is this bad?
Exhaustive enums are typically fine, but a project which does
not wish to make a stability commitment around exported enums may wish to
disable them by default.

### Example
```
enum Foo {
    Bar,
    Baz
}
```
Use instead:
```
#[non_exhaustive]
enum Foo {
    Bar,
    Baz
}
```