Don't return when not reading any input events

[TimonPost]

Attempt to fix crossterm-rs/crossterm#195

So this is an 'attempt' to fix this issue, altough I am not sure we should do something like this. I have checked the .Net framework source code out and they seem to do this the same way. This could be a temporary fix until we are going to refactor the input crate.

[matprec]

IMO the .Net implementation does something slightly different, they loop on events they choose not to handle while the proposed implementation busy-loops over unavaible input.

I've left some comments, i think we can replicate their behaviour quiet closely:

[TimonPost]

Ahhh, so if we remove that 0 check, it will start a read operation, but because there is no input, it will block read right? And thus no need for any kind of loop.

[matprec]

Ahhh, so if we remove that 0 check, it will start a read operation, but because there is no input, it will block read right?

Exactly!

And thus no need for any kind of loop.

I beg to differ - this way, the iterator will still yield None values for unknown events even though known event might occure later, which will short circuit patterns like this:

for _ in input.read_sync() {
    println!("Hello World!");
}

Looping internally in read_single_event until an known event occurs would enable such patterns.