Feedback from attendees

[hseeberger]

• Each lib.rs should have a module doc comment; if there is nothing noteworthy, at least mention that attendees shouls take a look at expectations.yml.
• "We do this by using ctr, the custom tool you just installed." in 01_better_assertions/00_intro.html is confusing, because it is not installed by the attendees, but instead provided as a binary crate. Mention this just as info and say that there is no need for the attendees to install anything.
• In 01/06, failed_subset_of() let x = btreeset![1, 2, 3]; needs to be let x = btreeset![&1, &2, &3];