file map doesn't work on Windows

[klutzy]

It seems that the code contains some bugs. At src.lib#L354-L358:

                let r = libc::MapViewOfFile(mapping,
                                            dwDesiredAccess,
                                            ((len as u64) >> 32) as DWORD,
                                            (offset & 0xffff_ffff) as DWORD,
                                            0);

Here len as u64 should be offset as u64. Also, the len value should be similarly passed to CreateFileMappingW at L342-L347.