* Make ConvertToAst trait fallible
It's got some panics, and we'll be switching those to errors!
* First example of a diagnostic-driven error
Add a diagnostic-driven error `#[wasm_bindgen]` being attached to public
functions, and add some macros to boot to make it easier to generate errors!
* Result-ify `src/parser.rs`
This commit converts all of `src/parser.rs` away from panics to using
`Diagnostic` instead. Along the way this adds a test case per changed `panic!`,
ensuring that we don't regress in these areas!