Compiler feedback

Rust is learned with the compiler, not around it. Before ownership starts, learners need the habit of reading error[E...], spans, notes, and help: suggestions. Planned exercises: 1. identify the primary error line. 2. recognise a help: suggestion. 3. choose the fix for a type mismatch. 4. fill a corrected variable name. 5. fill a corrected type. 6. apply a compiler hint to one line. 7. explain why a warning is not an error. 8. fix a short program from a rustc diagnostic. 9. use the diagnostic to repair a function.