"Generate getter" assist unnecessarily uses .as_ref() when deref coercion would work

[jonas-schievink]

struct S {
    v$0: Vec<u8>,
}

Generates:

impl S {
    fn v(&self) -> &[u8] {
        self.v.as_ref()
    }
}

It would probably be better if the body of v was just &self.v instead of going through .as_ref().