Respect None-delimited group when parsing unary expr

[dtolnay]

Example input: $expr.method() where expr is &self.

Before: incorrect, this is &(self.method())

Expr::Reference {
    expr: Expr::MethodCall {
        receiver: Expr::Path {
            path: Path {
                segments: [
                    PathSegment {
                        ident: "self",
                    },
                ],
            },
        },
        method: "method",
    },
}

After: correct. (&self).method()

Expr::MethodCall {
    receiver: Expr::Group {
        expr: Expr::Reference {
            expr: Expr::Path {
                path: Path {
                    segments: [
                        PathSegment {
                            ident: "self",
                        },
                    ],
                },
            },
        },
    },
    method: "method",
}