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/ui-tests/non-public-function.stderr

@@ -1,10 +1,8 @@
-error: custom attribute panicked
- --> $DIR/non-public-function.rs:7:1
+error: can only #[wasm_bindgen] public functions
+ --> $DIR/non-public-function.rs:8:1
   |
-7 | #[wasm_bindgen]
-  | ^^^^^^^^^^^^^^^
-  |
-  = help: message: can only bindgen public functions
+8 | fn foo() {}
+  | ^^^^^^^^^^^
 
 error: aborting due to previous error