Fix syntax in -Zunpretty-expanded output for derived PartialEq.

[nnethercote]

If you do derive(PartialEq) on a packed struct, the output shown by -Zunpretty=expanded includes expressions like this:

{ self.x } == { other.x }

This is invalid syntax. This doesn't break compilation, because the AST nodes are constructed within the compiler. But it does mean anyone using -Zunpretty=expanded output as a guide for hand-written impls could get a nasty surprise.

This commit fixes things by instead using this form:

({ self.x }) == ({ other.x })

r? @RalfJung