Cannot call fsync on tokio::fs::File

[tsturzl]

Hey there, first of all I have to say great work with this project. It's probably one of the most intuitive uring libraries out there. I've been working on a project using Rio along with Tokio. I'm using Rio both for file and network IO, and I noticed that most functions such as write_at take any type which implements AsRawFd, however some functions such as fsync explicitly take the std::fs::File type. I'm currently using tokio's File type(tokio::fs::File), since there are some file operations like retrieving metadata that I'd like to do asynchronously which isn't supported by Rio. I'm assuming you only accept a std::fs::File since you'd only ever expect to fsync a file rather than a socket, however this means I have to do some ugliness to use a tokio File type. Currently my work around for this is to grab the raw file descriptor from the tokio file and create a std file out of that descriptor, I don't believe this is any less safe than what Rio is doing, but it's kind of a pain. I'm wondering if you'd accept a PR that changed the couple of functions that expect Files to allow any type that implements AsRawFd, or if you find it concerning that this is potentially less safe or prone to misuse since then you can technically pass in something like a socket.