Rework OS abstractions

- As much as possible avoid dragging system headers into translation
  units. This should avoid namespace pollution and, hopefully, speed up
  builds a little bit.

- Avoid leaning on the UCRT so much on Windows - prefer native win32
  calls and native file handles except where doing so is inconvenient
  (in particular, for stat(), which we might try and replace later).

- Also, switch from SystemFunction036 to ProcessPrng on Windows. This
  requires us to generate a stub for bcryptprimitives.dll because
  Microsoft haven't bothered to provide a link library, but the function
  is better-documented and seems to be a more direct under-the-hood call
  as well. Apparently it's what's used by the major web browsers these
  days, which seems like a good indication it's stable and trusted.

- Lastly, remove a bunch of functions and macros and stuff that weren't
  actually being used. It seems good to try and keep the scope of
  OS-dependent stuff relatively contained and only add to it when
  actually required.

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