Introduce a sleep!() macro

[felix.s]

Tangentially related:

Add plain-simple way (in the standard library) for reading values from `stdin`?

Summary: add fn read_input<T: FromStr>(message: &str) -> T; to allow programming beginners to very easily read a value from stdin (and prompt the user beforehand). This would automatically deal with all errors and difficulties. Movitation Over the last couple of years, I worked a lot with programming beginners, i.e. people just learning how to program. I also held a lecture on Rust. Based on these experiences, I would really like to see a particular feature in Rust's standard library: easily r…