A calculator with a real parser

Build the front end of a programming language in miniature. A tokenizer that borrows from the input, a recursive-descent parser with precedence and parentheses, variables, and error messages that point at the exact spot.

Milestones

1. A tokenizer that borrows
2. Precedence with recursive descent
3. Parentheses and unary minus
4. Variables, and a second lifetime
5. Polish: errors that point at the problem