Result-ify src/parser.rs (#608)

* 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!

crates/macro-support/src/lib.rs

@@ -7,6 +7,7 @@ extern crate proc_macro2;
 extern crate quote;
 #[macro_use]
 extern crate syn;
+#[macro_use]
 extern crate wasm_bindgen_backend as backend;
 extern crate wasm_bindgen_shared as shared;