Support for async_std types?

[gabrik]

First, thank you for keeping alive this very useful crate.

I was wondering if there is any plan to support async_std types.
Especially in the MmapOptions.

As mentioned in the doc https://docs.rs/async-std/1.10.0/async_std/fs/index.html those types are just the async version of the ones in the standard library, so I guess it should be quite easy.

Thanks

[RazrFalcon]

Only if someone will send a patch.

[gabrik]

Sounds reasonable, do you would like to have the async_std support as a feature?
I think it would be better and in this way only people using async_std would activate it.

[RazrFalcon]

Fixed by #31