* Handle pub(restricted)
This commit properly handles pub(restricted) as introduced in RFC 1422
[0]. The syntax support was added in #971, but they were not correctly
formatted.
[0] https://github.com/rust-lang/rfcs/blob/master/text/1422-pub-restricted.mdFixes#970
* Drop #[inline] attribute on format_visibility
* Make newly non-failing functions return String
The change to `format_visibiilty` means that `format_header` and
`format_unit_struct` can no longer fail. Their return type is updated to
reflect that.
