Rollup merge of #101798 - y86-dev:const_waker, r=lcnr

Make `from_waker`, `waker` and `from_raw` unstably `const`

Make
- `Context::from_waker`
- `Context::waker`
- `Waker::from_raw`

`const`.

Also added a small test.

library/std/src/lib.rs

@@ -315,6 +315,7 @@
 #![feature(strict_provenance)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(const_maybe_uninit_uninit_array)]
+#![feature(const_waker)]
 //
 // Library features (alloc):
 #![feature(alloc_layout_extra)]