Reorganize the "Source Code Representation" chapters

[BoxyUwU]

This ticks off a number of boxes from #2141, specifically it:

• Creates a new chapter Supporting Infrastructure and moves a number of unrelated chapters out of the source code repr chapter
• Removes the chapter about IR IDs moving its information into the respective IR chapters
• Removes a number of type definitions from the inline asm chapter, instead opting to reference the rustc

The only remaining follow ups would be to flesh out the hir chapter some more, and talk about async stuff somewhere. It might be worth moving those to their own issues though

cc @jieyouxu

[jieyouxu]

Thanks! Very nice cleanup 👍

[jieyouxu]

flesh out the hir chapter some more, and talk about async stuff somewhere. It might be worth moving those to their own issues though

I'll split the issues.