core: add `unsafe_memoryrefload` and `unsafe_memoryrefstore!`
note: CI is running at the _top_ of the stack
in my third attempt at https://github.com/JuliaLang/julia/issues/51658 and https://github.com/JuliaLang/julia/issues/48801, I'm trying to go this route of a new builtin for fast reinterpreted reads from the underlying `Memory`, letting us avoid the `cconvert` + `unsafe_load` to begin with. that was problematic because it required a `GC.@preserve` which blocked vectorization.
even though `Memory` already allows `Ptr` access, any `unsafe_load/store`s through it yield very unhelpful aliasinfo since it might touch the array header which means it then can't be hoisted above the loop. thus this builtin could exist to allow:
* reading the bits more or less raw from a `Memory` (here phrased as reading an `isbits` type `T`)
* preserving the memory from GC while doing so
* but also promising that the access remains inside the `Memory`'s data and not its header
this is my first time using the stacked-PRs feature but the intention is to show that this one adds the core functionality, and the PRs on top are sample use cases (the `ReinterpretArray` one being the most salient)
Assisted-by: Codex (GPT 6.1 Sol)
Assisted-by: Claude Code (Claude Opus 5.5)
