Coerced app_data can hard crash

[kurtlawrence]

Take example below. Crashes, but using &String instead of &str works. Maybe something to do with coercion, and might be a little tricky solve =/.

let v = String::from("Hello, world!");

let mut data = repl_data!(&str);
let mut repl = papyrus::Repl::default_terminal(&mut data);

for ch in "app_data\n".chars() {
    repl = match repl.push_input(ch) {
        papyrus::repl::PushResult::Read(r) => r,
        papyrus::repl::PushResult::Eval(r) => r.eval(&v).unwrap().print(),
    }
}