Auto merge of #137425 - yotamofek:pr/rustdoc/return-impl-display-redux, r=GuillaumeGomez

`librustdoc`: return `impl fmt::Display` in more places instead of writing to strings

Continuation of #136784 , another attempt at landing the larger parts of #136748 .
I'd like to, gradually, make all of the building blocks for rendering docs in `librustdoc` return `impl fmt::Display` instead of returning `Strings`, or receiving a `&mut String` (or `&mut impl fmt::Write`). Another smaller end goal is to be able to get rid of [`write_str`](https://github.com/rust-lang/rust/blob/8dac72bb1d12b2649acd0c190e41524f83da5683/src/librustdoc/html/format.rs#L40-L42).
This PR is a large step in that direction.

Most of the changes are quite mechanical, and split up into separate commits for easier reviewing (hopefully). I took `print_item` and then started by converting all the functions it called (and their dependencies), and the last commit does the conversion for `print_item` itself. Ignoring whitespace should make reviewing a bit easier.

And most importantly, perf run shows pretty good results locally, hopefully CI will also show green 😁

r? `@GuillaumeGomez` , if you feel like it.

Author: bors