Use vertical layout for complex attributes

[topecongiro]

Currently rustfmt forces mixed layout when formatting attributes. This is suboptimal when the attribute has complex inner items. This PR fixes it by disallowing mixed layout when every item in the attribute is either list or assignment.

[nrc]

Thanks for the PR! I think that we should use block formatting rather than visual formatting though, e.g.,

#![doc(
    html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
    html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
    html_root_url = "https://doc.rust-lang.org/nightly/",
    html_playground_url = "https://play.rust-lang.org/",
    test(attr(deny(warnings))),
)]

(Although I think we need to follow macros and not insert a trailing comma if there is not one there already, since in the future it could be significant).

I think requiring all items in the list to be complex is probably to weak - e.g., imagine an attribute like the above example but with a trailing 0 - we'd still want to multi-line it, IMO. We could either have an any condition, rather than all or a 'more than half' condition (or something like that) or we could have something based on the overall length of the attribute - like always use vertical formatting if it goes multi-line.

[topecongiro]

@nrc Thank you for your review. Actually the current implementation does the opposite of what I said at the top - it uses mixed layout only when all the items are 'simple'. My appologies for wrong description.

[topecongiro]

I have updated the PR to use indent_style when formatting attributes, so that we use block indent style by deafult.

[nrc]

Thanks!