Implement AsRawFd, IntoRawFd, AsRawHandle, IntoRawHandle and Into<Stdio> for MMap about memmap2-rs

[RazrFalcon]

Patches are welcome.

[adamreichold]

I don't the example can work as it is: An anonymous mapping is not backed by a file descriptor (by definition as it is anonymous).

When a mapping is backed by a file, we take that file by reference, c.f. MmapAsRawDesc and hence you can use that for `Command´ directly.

memfile explicitly starts with a memory-backed file created via memfd_create (which is not available on Windows AFAIK), i.e. you could use a MemFile to back our Mmap, but an anonymous mapping does not necessarily have a file descriptor associated at all.