`AwkwardArray.is_valid` is incomplete

[jpivarski]

It doesn't check rules like, "thou mustn't include an indexed or option-type array directly within another indexed or option-type array" and the complex rules surrounding union composition. (UnionArrays cannot be directly within an indexed or option-type array, and if it's a union of option-type data, the first element of contents must be option-type while all the rest must be UnmaskedArrays.)

I should review all of the rules and encode them into the is_valid function in Julia, so that anything that passes is_valid can be sent to Python without error.