Rollup merge of #124049 - slanterns:const_io_structs_stabilize, r=jhp…

…ratt

Stabilize `const_io_structs`

This PR stabilizes `const_io_structs`.

Tracking issue: rust-lang/rust#78812.
Implementation PR: rust-lang/rust#78811.

FCPs already completed in the tracking issue.

Closes rust-lang/rust#78812.

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

r? libs-api