Coding Matrix in Rust | 2023-12-04

This time: * Allow deleting greetings (fixed the bug where tests froze because of a lock in test code) * Renamed "greeter" to "greet" * Got properly confused trying to pause for a random time before greeting Next time: * Figure out how to provide a random number to the callback function (so we can sleep for a random time) without needing the schedule to be `Sync` * Or in other words, try to become less confused (briefly, I imagine)