Add Thread::into_raw and Thread::from_raw functions

[h33p]

Allows converting a Thread into a raw pointer for use in futures executors, FFI and other lower level code.

ACP: rust-lang/libs-team#200

I took the liberty to assign thread_pointer_conversion feature flag, but I'm not sure if the functions are the only place I need to add it.

Also I'm unsure whether from_raw needs #[must_use] or not.

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[h33p]

@rustbot label +T-libs-api -T-libs

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

........................................................................................ 792/1132 .................................................................................iii.... 880/1132 ........................................................................................ 968/1132 .............................................................................iiiiii..... 1056/1132 .......................F.F..................i............................... failures: ---- src/thread/mod.rs - thread::Thread::from_raw (line 1346) stdout ---- error[E0658]: use of unstable library feature 'thread_pointer_conversion' --- | ^^^^^^^^ | = help: add `#![feature(thread_pointer_conversion)]` to the crate attributes to enable error[E0277]: `*const ()` cannot be sent between threads safely | | 9 | let handle = thread::spawn(move || { | ^^^^^^^^^^^^^ `*const ()` cannot be sent between threads safely | = help: within `(*const (), *const ())`, the trait `Send` is not implemented for `*const ()` = note: required because it appears within the type `(*const (), *const ())` note: required by a bound in `spawn` | 680 | T: Send + 'static, | ^^^^ required by this bound in `spawn` --- | ^^^^^^^^^^^^^^^^ | = help: add `#![feature(thread_pointer_conversion)]` to the crate attributes to enable error[E0369]: binary operation `==` cannot be applied to type `Thread` | | 20 | assert_ne!(main_thread, child_thread); | | | Thread | Thread | | = note: this error originates in the macro `assert_ne` (in Nightly builds, run with -Z macro-backtrace for more info) error[E0369]: binary operation `==` cannot be applied to type `Thread` | 21 | assert_eq!(main_thread, thread::current()); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | | --- src/thread/mod.rs - thread::Thread::into_raw (line 1316) test result: FAILED. 1112 passed; 2 failed; 18 ignored; 0 measured; 0 filtered out; finished in 11.61s error: doctest failed, to rerun pass `-p std --doc` 

[h33p]

Duplicate of #97524