Rollup merge of #83065 - CDirkx:win-alloc, r=dtolnay

Rework `std::sys::windows::alloc`

I came across https://github.com/rust-lang/rust/pull/76676#discussion_r488729990, which points out that there was unsound code in the Windows alloc code, creating a &mut to possibly uninitialized memory. I reworked the code so that that particular issue does not occur anymore, and started adding more documentation and safety comments.

Full list of changes:
 - moved and documented the relevant Windows Heap API functions
 - refactor `allocate_with_flags` to `allocate` (and remove the other helper functions), which now takes just a `bool` if the memory should be zeroed
 - add checks for if `GetProcessHeap` returned null
 - add a test that checks if the size and alignment of a `Header` are indeed <= `MIN_ALIGN`
 - add `#![deny(unsafe_op_in_unsafe_fn)]` and the necessary unsafe blocks with safety comments

I feel like I may have overdone the documenting, the unsoundness fix is the most important part; I could spit this PR up in separate parts.

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