review meta code

Stream: mathlib4

[Floris van Doorn]

Can I ask someone to review the meta code I wrote last week to to_additive and simps? !4#1779 !4#1780 !4#1819 !4#1861 !4#1881 !4#1885.
Some of them are PRs that require refactors throughout the library, so they currently have merge conflicts or build errors, but I'm not going to fix the conflicts every time they occur, since it's much easier to do them all at once when the PR is delegated.