Rustdoc: JSON backend experimental impl, with new tests.

[aDotInTheVoid]

Based on #75114 by @P1n3appl3

The first commit is all of #75114, but squased to 1 commit, as that was much easier to rebase onto master.

The git history is a mess, but I think I'll edit it after review, so it's obvious whats new.

Still to do

• Update docs.
• Add bless option to tests.
• Add test option for multiple files in same crate.
• Decide if the tests should check for json to be equal or subset.
• Go through the rest of the review for the original pr. (This is open because the test system is done(ish), but stuff like not using a hashmap and using CRATE_DEF_INDEX hasn't)

I'm also sure how many of these we need to do before landing on nightly, as it would be nice to get this in tree, so it isn't effected by churn like #79125, #79041, #79061

r? @jyn514