Rollup merge of #108161 - WaffleLapkin:const_param_ty, r=BoxyUwU

Add `ConstParamTy` trait

This is a bit sketch, but idk.
r? `@BoxyUwU`

Yet to be done:
- [x] ~~Figure out if it's okay to implement `StructuralEq` for primitives / possibly remove their special casing~~ (it should be okay, but maybe not in this PR...)
- [ ] Maybe refactor the code a little bit
- [x] Use a macro to make impls a bit nicer

Future work:
- [ ] Actually™ use the trait when checking if a `const` generic type is allowed
- [ ] _Really_ refactor the surrounding code
- [ ] Refactor `marker.rs` into multiple modules for each "theme" of markers

library/core/src/lib.rs

@@ -162,6 +162,7 @@
 #![feature(const_waker)]
 #![feature(core_panic)]
 #![feature(duration_consts_float)]
+#![feature(internal_impls_macro)]
 #![feature(ip)]
 #![feature(is_ascii_octdigit)]
 #![feature(maybe_uninit_uninit_array)]