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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If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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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
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.............................................................................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