Update code examples

[giacomocavalieri]

This PR updates the stdlib code examples:

• Now they all use assert rather than comments to show expected values (with the exception of functions producing non deterministic values)
• The assert examples are formatted as if they were formatted by the Gleam formatter (to the best of my ability, something might have slipped! 😁)
• Now there's always a single assertion per block
• References to the deprecated list.range have been removed from all doc comments

[lpil]

Thanks Jak!!!