unnamed fields using `contents` generate compiler errors
when I use something like `- contents: [ 1, 2, 3 ]` in a ksy file, since the 0.9.0 release, I get the following error: ```unable to access 'unnamed_0' in test_ksy context```.

This used to work fine before the 0.9.0 release and is easily reproducable with https://ide.kaitai.io.