rustdoc DocFragment rework

[GuillaumeGomez]

Kind of a follow-up of #80119.

A few things are happening in this PR. I'm not sure about the impact on perf though so I'm very interested about that too (if the perf is worse, then we can just close this PR).

The idea here is mostly about reducing the memory usage by relying even more on Symbol instead of String. The only issue is that DocFragment has 3 modifications performed on it:

1. Unindenting
2. Collapsing similar comments into one
3. "Beautifying" (weird JS-like comments handling).

To do so, I saved the information about unindent and the "collapse" is now on-demand (which is why I'm not sure the perf will be better, it has to be run multiple times...).

r? @jyn514