Handle recursive input objects

[obmarg]

This function infinitely recurses if it finds an input object that contains itself: https://github.com/obmarg/cynic/blob/master/cynic-querygen/src/query_parsing/inputs.rs#L181-L211

[obmarg]

I assume the fix for this is to be smarter about this set here - it could potentially be a HashMap or similar, with a lookup to avoid the recursion.