Extend MmapAsRawDesc to handle anything implementing AsRawFd

[adamreichold]

This automatically extends support to e.g. async-std. For example, the following variant on its cat example works

use std::env::args;

use async_std::fs::File;
use async_std::io;
use async_std::prelude::*;
use async_std::task;

use memmap2::Mmap;

fn main() -> io::Result<()> {
    let path = args().nth(1).expect("missing path argument");

    task::block_on(async {
        let file = File::open(&path).await?;
        let map = unsafe { Mmap::map(&file)? };

        let mut stdout = io::stdout();
        stdout.write_all(&map).await?;
        stdout.flush().await?;

        Ok(())
    })
}

[adamreichold]

I am looking into doing the same for Windows, i.e. going for

#[cfg(windows)]
pub struct MmapRawDescriptor(RawHandle);

even though it will mean breaking this up against #[cfg(not(any(unix, windows)))] again. Sorry for the back and forth... |-\

[RazrFalcon]

Sure, no problem. I like those patches.