Tracking issue for dejargonification

[oli-obk]

There are some things in rustc that are named historically or just really oddly. This issue tracks renaming them to reflect what they are without having to understand Rust's history and/or programming language design/theory jargon.

• Region -> Lifetime in general. While Region kinda makes more sense, we use Lifetime in the language
• EarlyBoundRegion -> GenericLifetime (to reflect what they are in the language)
• LateBoundRegion ->HigherRankedLifetime (")
• *Folder and fold_* -> *Replacer and replace_*
• update internal terminology: Substs -> GenericArgs #110793
• subst -> replace_generics

These changes have an accepted MCP at rust-lang/compiler-team#451

Note: please do not pick up any of these work items without first checking in with me. These renamings aren't universally liked and we should make sure everyone stays on board.

I'm not gonna move on these things quickly, and I'm fine closing PRs even after putting in lotsa work. So if anyone has concerns, please raise them on a zulip thread or directly with me.

[lqd]

Maybe we can also finally pick one of "free region" vs "universal region", or something else t-types would prefer. (IIRC sometimes there's even both names used in the same function :)

[Noratrieb]

If you want to discuss the naming, use this zulip thread instead: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/naming.20bikeshedding