rustdoc: sidebar generation code duplicates logic

[lolbinarycat]

I recently discovered this in #138574, when I fixed the bug for the main page, but the erroneous items were still in the sidebar.

If we simply collected a list of sidebar items during rendering of the main page, almost the entirety of sidebar.rs could be removed. This could even be done incrementally, with each sidebar_* function being replaced with a field on Context.

[yotamofek]

IIUC, it would also mean we won't need interior mutability for deref_id_map, which might improve performance by a little bit.
And consequently, might also allow us to "lazily" render the content and sidebar into the same buffer, which might improve perf, maybe even substantially.

So - good idea! LMK if I can help out with this.

[lolbinarycat]

I'm thinking we'll add a field to Context, like sidebar_map: HashMap<ItemId, SidebarLinks>, where SidebarLinks is a struct that has fields for each different type of sidebar item.

The one downside is that Link will likely be unable to effectively use Cow, which will have some impact due to some extra cloning being required.

We'll have to do some profiling to see if not recalculating things makes up for the extra allocations.