This was originally intended to just change `swap` to `compare_exchange`
however other changes were needed too:

* Load with `Release` ordering seemed wrong because if it loaded `DONE`
  before while loop it wouldn't synchronize with the storing thread.
* There's no point in repeatedly checking if the value is `NONE` so this
  check was moved about while loop
* Due to a mistake I wanted to inspect using `miri` but couldn't because
  of FFI call. Separating the once implementation from consumer allowed
  writing a test in pure Rust and inspect via `miri`.
* The initializing code now skips loading the value again because it
  already knows it's initialized.
* Orderings are now explained in the comments

This caches the initialization state of preallocated context in a thread
local variable to avoid atomics in subsequent accesses.

Accessing `std::mem` instead of `core::mem` caused fuzzing to break
without `std`.

This reverts commit 5cbf5ee.