Improve UI/UX for trait implementors async loading

In #142380, we made changes that made negative trait implementors to be listed first, which means that async implementors can appear while the user is looking at this list, making the UX not great. To solve this, we decided to hide all implementors and only make them appear once the async loading is done, which also can be surprising.

So this issue mostly serves as a place where to debate how we could do this better.

cc @rust-lang/rustdoc-frontend