Remove unnecessary sigils and `ref`s in derived code.

E.g. improving code like this:
```
match &*self {
    &Enum1::Single { x: ref __self_0 } => {
        ::core::hash::Hash::hash(&*__self_0, state)
    }
}
```
to this:
```
match self {
    Enum1::Single { x: __self_0 } => {
        ::core::hash::Hash::hash(&*__self_0, state)
    }
}
```
by removing the `&*`, the `&`, and the `ref`.

I suspect the current generated code predates deref-coercion.

The commit also gets rid of `use_temporaries`, instead passing around
`always_copy`, which makes things a little clearer. And it fixes up some
comments.

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