RFC: support mmap on Windows

I don't have a build environment set up for Windows, so I haven't tested this directly. However, I tested the "ingredients" successfully in a standalone file, so the odds are not entirely zero for this working. However, if it's not practical for anyone else to test this, I can (with some time) give that a try.

It does pass tests on Linux; I should mention that I have seen some test failures, but they don't seem to be consistent, and I got this to pass testall three times in a row before submitting.