rustdoc: implementations are starting contracted

[AaronKutch]

I updated Rust nightly to rustc 1.53.0-nightly (42816d61e 2021-04-24) after a long break and found that some recent change to rustdoc is making all my documentation start contracted. I have this complicated crate with several impl blocks spread across several files, and previously the documentation would look like this:

but now looks like this:

If I want to quickly scan through all the sections, I have to click every single section and expand them every time I open up that documentation page. I think I could see a reason for changing the non-trait impls to also start contracted as default, but now there are no bold titles delimiting the different sections (Miscellanious, Multiplication, Division, etc). I think there needs to be some kind of attribute to make impls expanded by default and/or some way to hoist the impl titles on the different impls.

[jyn514]

Fixed by #84552