Support `Mmap*::lock` on Windows

[DrChat]

Windows supports locking virtual memory into physical memory via the VirtualLock function (and conversely VirtualUnlock).

One crucial difference from Unix is that you must first ensure that the process working set size is large enough to accommodate the memory region via SetProcessWorkingSetSize.
The easiest solution here would be to just leave it up to callers to configure the working set size (and giving them a reminder to do so in the documentation).

[de-vri-es]

The limit is pretty similar to the RLIMIT_MEMLOCK on Linux. POSIX itself only mentions a "implementation defined limit". So it seems that the behaviour of VirtualLock() is pretty much compatible with mlock() \o/