moving Pin API to a seperate module

[nivkner]

@withoutboats proposed in #49150 to move the Pin API to its own module in order to have a place to provide a high level overview for it.
I am making this a separate issue to discuss this particular aspect of the stabilization process, to not overload the tracking issue.

I found a problem with trying to move the API:
both the Option type and Future trait in core depend on PinMut directly so it can't just be moved from core, and PinBox requires allocation so it can't be in core.

should those types just be re-exportd from core and alloc into a module for the sake of unified documentation then? or is there some other way?

[cramertj]

@nivkner It sounds like there's some confusion-- I think the discussion was about moving them into their own module (e.g. mod pin), not their own crate. They would still be defined in the core and alloc crates.

[withoutboats]

@nikver with the way core/std are currently set up, the way to do this is:

1. Put PinMut in a module called pin in libcore.
2. In libstd, create a pin module with PinBox and pub use core::pin::*; in it.

EDIT: Sorry I guess PinBox should be in alloc::pin.

[nivkner]

implemented the change described here in #53227, so closing this, thanks.