Exercise: Rewriting with Result
The following implements a very simple parser for an expression language.
However, it handles errors by panicking. Rewrite it to instead use idiomatic
error handling and propagate errors to a return from main. Feel free to use
thiserror and anyhow.
HINT: start by fixing error handling in the parse function. Once that is
working correctly, update Tokenizer to implement
Iterator<Item=Result<Token, TokenizerError>> and handle that in the parser.