Strings and slices

Rust has owned strings (String) and borrowed string slices (&str). The same borrowed-view idea applies to slices like &[T]. This theme teaches the signature reflex: accept &str or &[T] when you only read, return String or Vec<T> when you create owned data. Planned exercises: 1. choose &str for a read-only parameter. 2. choose String for owned output. 3. recognise slice syntax. 4. fill .to_string(). 5. fill a slice parameter. 6. write a string-reading helper. 7. write a slice-summing helper. 8. translate TS string code. 9. fix a function that returns a reference to a temporary string.