Option

This example uses a lot of lists of known length, but with the introduction of list::get a lot of options are added to the flow of logic. A way to mark impossible branches is needed.

This is handled using a new external function called std::panic. Since Orchid is a sandboxed language this doesn't actually cause a Rust panic, instead it produces a dedicated ExternError when it's first reduced. Using this, option::unwrap is trivial to define.