lint `Hash` impls with `match`es on enums but no `mem::discriminant` call

Implementing Hash by hashing just the fields of enum variants may cause hash collisions. E.g.

enum Foo {
    A(i32),
    B(i32),
}
impl Hash for Foo {
    fn hash<H: Hasher>(&self, hasher: &mut H) {
        match self {
             Foo::A(i) => i.hash(hasher),
             Foo::B(i) => i.hash(hasher),
        }
    }
}

would only hash the i32s, which makes the hash of Foo::A(42) equal to the hash of Foo::B(42). What's missing here is a call to mem::discriminant(self).hash(hasher); to differentiate between the variants.

This will lead to false positives if the user is doing some sort of manual scheme to differentiate between the variants, but in general the discriminant scheme should be preferred.

The issue names no source file or test. Start from the Rust enum Hash example and locate the relevant Clippy lint entry point and test conventions; done means detecting matches that hash variant fields without hashing mem::discriminant, while accounting for the noted false positives.