improve SymbolDef info in `--output ast` (#2778)...* SymbolDef info (a.k.a. `thedef`) is now represented as a string containing `"ID name [mangled_name]"`.
* Enhance display of `globals`, `variables`, `functions` and `enclosed`.
* `SymbolDef.next_id` starts at `1` and the `id` is adjusted for `-o ast` display.