impl Trait and trait objects

Rust has two styles of polymorphism: static dispatch with generics or impl Trait, and dynamic dispatch with dyn Trait. This theme teaches when each is appropriate and why trait objects usually live behind a pointer such as &dyn Trait or Box<dyn Trait>. Planned exercises: 1. choose impl Trait for an argument. 2. recognise dyn Trait. 3. choose Box<dyn Trait> for heterogeneous owned values. 4. fill an impl Iterator return. 5. fill &dyn Display. 6. write a static dispatch helper. 7. write a trait-object collection. 8. translate TS interface polymorphism. 9. fix an attempt to return two concrete types from one impl Trait function.