Closures

Rust closures look like compact TypeScript arrow functions, but they capture by borrow, mutable borrow, or move depending on use. Planned exercises: 1. choose closure syntax. 2. recognise inferred parameter types. 3. choose move only when ownership must transfer. 4. fill a predicate closure. 5. fill a mapping closure. 6. write a closure used with sort_by_key. 7. write a closure that captures a threshold. 8. translate TS arrow functions in array methods. 9. fix a closure capture that conflicts with a later borrow.