Use stable AST IDs

Instead of simple numbering, we hash important bits, like the name of the item.

This will allow for much better incrementality, e.g. when you add an item. Currently, this invalidates the IDs of all following items, which invalidates pretty much everything.

crates/syntax/src/ast/node_ext.rs

@@ -30,6 +30,16 @@ impl ast::Name {
     pub fn text(&self) -> TokenText<'_> {
         text_of_first_token(self.syntax())
     }
+    pub fn text_non_mutable(&self) -> &str {
+        fn first_token(green_ref: &GreenNodeData) -> &GreenTokenData {
+            green_ref.children().next().and_then(NodeOrToken::into_token).unwrap()
+        }
+
+        match self.syntax().green() {
+            Cow::Borrowed(green_ref) => first_token(green_ref).text(),
+            Cow::Owned(_) => unreachable!(),
+        }
+    }
 }
 
 impl ast::NameRef {