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/backend/src/error.rs

@@ -1,6 +1,20 @@
 use proc_macro2::*;
 use quote::ToTokens;
 
+#[macro_export]
+macro_rules! err_span {
+    ($span:expr, $($msg:tt)*) => (
+        ::backend::Diagnostic::span_error(&$span, format!($($msg)*))
+    )
+}
+
+#[macro_export]
+macro_rules! bail_span {
+    ($($t:tt)*) => (
+        return Err(err_span!($($t)*).into())
+    )
+}
+
 pub struct Diagnostic {
     inner: Repr,
 }