Implement REPL package with Run(io.Reader, io.Writer)
- Read-eval-print loop: reads lines, tokenizes, parses, evaluates, prints result
- Prints prompt '>> ' before each input
- Error handling: prints 'error: <msg>' and continues loop
- Skips empty lines
- Formats results with %g (clean integers, no trailing zeros)
- 10 unit tests covering: single/multiple expressions, error recovery,
division by zero, empty lines, prompt display, float results,
invalid chars, empty input, integer formatting
2 files changed