evaluator: support fixed_size in variable position and add tests

[Mrmaxmeier]

Hi,

I tried using Field field [[fixed_size(..)]]; and got unexpected results with the next field's position. It seems like this use-case was never handled as the position is reset in applyVariableAttributes's scope guard after being updated in applyTypeAttributes.

I've added a test with how I expected it to work. Let me know if that's reasonable :)

Thanks!