[rustdoc] Add support new bang macro kinds

[GuillaumeGomez]

This implements #145153 in rustdoc. This PR voluntarily doesn't touch anything related to intra-doc links, I'll send a follow-up if needed.

So now about the implementation itself: this is a weird case where a macro can be different things at once but still only gets one file generated. I saw two ways to implement this:

1. Handle ItemKind::Macro differently and iter through its MacroKinds values.
2. Add new placeholder variants in the ItemKind enum, which means that when we encounter them in rendering, we need to ignore them. It also makes the ItemKind enum bigger (and also needs more code to be handled). Another downside is that it needs to be handled in the JSON output.

I implemented 1. in the first commit and 2. in the third commit so if we don't to keep 2., I can simply remove it. I personally have no preference for one or the other since the first is "simpler" but it's easy to forget to go through the macro kinds whereas the second needs a lot more code.

Now there was an interesting improvement that came with this PR in the html::render::print_item::item_module function: I simplified its implementation and split the different parts in a HashMap where the key is the item type. So then, we can just iterate through the keys and open/close the section at each iteration instead of keeping an Option<section> around.

cc @joshtriplett

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[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha, @lolbinarycat