Rollup merge of #60165 - Nemo157:pin-into-inner, r=cramertj

Add Pin::{into_inner,into_inner_unchecked}

These functions are useful for unsafe code that needs to temporarily pull smart pointers out of the `Pin`, e.g. [the change that inspired them](https://github.com/Nemo157/futures-rs/commit/b4361780faf764c7cb046ed75f863a6fcfd44800#diff-1a4e0ba4d1b539412ca576411ec6c7c2R258) is taking a `Pin<Box<dyn Future>>`, turning it into a `*mut dyn Future` via `Box::into_raw(unsafe { Pin::into_inner_unchecked(pin) })` then later dropping this via `drop(Pin::from(Box::from_raw(ptr)))`. This can be accomplished today via `{ let ptr = unsafe { Pin::get_unchecked_mut(pin.as_mut()) } as *mut dyn Future; mem::forget(pin); ptr }`, but this is far more complicated and loses out on the symmetry of using `Box::into_raw` and `Box::from_raw`.

I'll extend the documentation on what guarantees `into_inner_unchecked` needs to uphold once I get some feedback on whether this API is wanted or not.

r? @withoutboats

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