Add fuzzing to the parser.

As part of this, todos and panics where moved outside of this module
to elsewhere when they would cause fuzzing to fail.

compiler/parse/src/ast.rs

@@ -277,6 +277,8 @@ pub enum Def<'a> {
     /// This is used only to avoid cloning when reordering expressions (e.g. in desugar()).
     /// It lets us take a (&Def) and create a plain (Def) from it.
     Nested(&'a Def<'a>),
+
+    NotYetImplemented(&'static str),
 }
 
 #[derive(Debug, Clone, PartialEq)]