`impl_stable_hash_for!` does not support enums with named fields

[RalfJung]

In miri, we are now carrying some hand-written implementations of HashStable because the macro impl_stable_hash_for! does not support enum variants with named fields, e.g.

pub enum StackPopCleanup {
    Goto(Option<mir::BasicBlock>),
    None { cleanup: bool },
}

This is rather annoying and also error-prone, e.g. the manual implementations forgot to hash the discriminant.

impl_stable_hash_for! should support enums like the above.