make std::intrinsic functions actually be intrinsics

[RalfJung]

Most of the functions in std::intrinsics are actually intrinsics, but some are not: for historical reasons, std::intrinsics::{copy,copy_nonoverlapping,write_bytes} are accessible on stable, and the versions in std::ptr are just re-exports. These functions are not intrinsics, but wrappers around the intrinsic, because they add extra debug assertions.

This PR makes the functions in std::intrinsics actually be intrinsics.

• The advantage is that we can now use it in tests that need to directly call the intrinsic, thus removing a footgun for compiler development. We also remove the extended user-facing doc comments of these functions out of a file that should be largely internal documentation.
• The downside is that if users are using those functions directly, they will not get the debug assertions any more. Note however that those users are already ignoring a deprecation warning, so I think this is fine. Furthermore, if someone imports the intrinsic name of this function and turns that into a function pointer, that will no longer work, since only the wrapper functions can be turned into a function pointer. I would be rather surprised if anyone did this, though... and again, they must have already ignored a deprecation warning. Still, seems worth a crater run, if there's general agreement that we want to go ahead with this change.

(intrinsics::drop_in_place also remains not-an-intrinsic, which bugs me, but oh well, not much we can do about it; we can't remove it from the module as the path is accidentally-stable.)

Cc @rust-lang/libs-api @saethlin

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

The Miri subtree was changed

cc @rust-lang/miri

[jhpratt]

(intrinsics::drop_in_place also remains not-an-intrinsic, which bugs me, but oh well, not much we can do about it; we can't remove it from the module as the path is accidentally-stable.)

I'd be interested in seeing a crater run on that, to be honest. We've pulled the rug on things that were accidentally stabilized before, even long after the fact.