Add a simple repl example

[bjorn3]

It only supports a subset of the functionality of headcrab, but it is a good example of how to use headcrab.

[nbaksalyar]

This is very exciting, thanks! :)
If it's easier, the example can be disabled for macOS temporarily - we can create a separate issue to address it.

[bjorn3]

I was already looking for a way to disable it on macOS. I can't find any.

[nbaksalyar]

I can't find any.

One hack is to use an empty fn main for all targets other than Linux and put all other code into an internal module.

E.g.:

#[cfg(target_os = "linux")]
fn main() {
  example::main();
}

#[cfg(not(target_os = "linux"))]
fn main() {
  println!("This example is not supported for OSes other than Linux");
}

and then the example implementation can be hidden in an internal module in a similar way:

#[cfg(target_os = "linux")]
mod example {
  struct Context { ... }

  fn main() {
    // the actual main function
  }
}