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

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

### Example
```
struct Foo {
    bar: u8,
    baz: String,
}
```
Use instead:
```
#[non_exhaustive]
struct Foo {
    bar: u8,
    baz: String,
}
```