Auto merge of rust-lang#146992 - GuillaumeGomez:improve-highlight, r=yotamofek,lolbinarycat

Improve source code for `highlight.rs`

I was very bothered by the complexity of this file, in particular the handling of `pending_elems` which was very tricky to follow.

So instead, here comes a more sane approach: the content is store in a stack-like type which handles "levels" of HTML (ie, a macro expansion can contain other HTML tags which can themselves contain other, etc). Making it much simpler to keep track of what's going on.

r? `@lolbinarycat`

Author: bors