added some Win32 file memory mapping functions

[mondegreengames]

No description provided.

[squeek502]

Just a heads up: this is unlikely to be merged unless there's a use-case within the standard library or the Zig compiler source code.

Relevant issues:

• Prefer depending on NtDll rather than kernel32 or other higher level DLLs #1840
• reduce windows API bindings in the standard library to the minimum required for the standard library's own abstractions to work #4426

Note also that this does not have to be part of the standard library for you to be able to use it. These bindings can go in your project's source and it'll work just fine.

[mondegreengames]

Gotcha. This PR started when I found that Stat isn't defined in std\c\windows.zig. It just has this:

pub const Stat = @compileError("TODO windows Stat definition");

But that may be a more worthwhile PR. I'll close this one.