[tools] Allow struct types in definitions

Allow struct type names in definitions, which result in struct buffer
pointers in generated code.

assets/grammars/definitions.g

@@ -24,10 +24,12 @@ method: description? "method" name options? ("{" param* "}")?
 
 param: "param" name type options? description?
 
-?type: PRIMITIVE | object_name
+?type: PRIMITIVE | object_name | struct_name
 
 object_name: "ref" name
 
+struct_name: "struct" name
+
 id: NUMBER
 name: IDENTIFIER
 options: "[" ( OPTION | IDENTIFIER )+ "]"