Add ManuallyDrop lang item for precise control of destructors

[gereeter]

See rust-lang/rfcs#197.

cc @huonw

[Gankra]

Presumably this should be on get_mut, which actually doesn't seem to be implemented?

[huonw]

I would prefer the RFC get feedback from other core team members before we land that design. (I'm not putting any effort into pushing the RFC forward atm because it seems totally optional for 1.0.)

[gereeter]

I agree that it would be nice to get more feedback on the RFC and I understand that this isn't necessary for 1.0. However:

• The RFC has received nothing but positive responses.
• The RFC has been open and waiting for comments for several months.
• The change involved is tiny (3 lines to the compiler) and should have no adverse side effects.
• I keep running in to situations in which I have to fall back to heap allocation because I don't have access to ManuallyDrop. The specific instance that prompted this PR was me trying to optimize BTreeMap's iterators to render Add intrusive iterators to BTree. #19796 unnecessary and needing ManuallyDrop to avoid allocation during iteration. I'd really like to avoid having extra functions like intrusive iterators in 1.0 Rust, especially not marked #[stable], unless there is no good way to optimize the idiomatic alternatives.
• I'm only exposing ManuallyDrop under core, not std.
• All parts of ManuallyDrop are marked #[experimental].

[Gankra]

RFC hasn't moved and nothing has been happening here. Closing due to inactivity; sorry 😢