feat(smith): add DocumentBuilder::input_exhausted

[SimonSapin]

This indicates we might have run into rust-fuzz/arbitrary#139

[SimonSapin]

My current attempt at working around rust-fuzz/arbitrary#139 is to double the size of the input until it’s large enough to not be exhausted, but currently this causes DocumentBuilder::operation_definition to overflow the stack, I suspect through generating very deeply nested selection sets.

[SimonSapin]

By the way, at first this PR seems unnecessary because DocumentBuilder only borrows &mut Unstructured<'_> so after it is dropped the called should be able to call Unstructured::is_empty themselves. However because DocumentBuilder only has one lifetime parameter and borrows &'a mut Unstructured<'a> with the same 'a in two places. Because something something variance, Unstructured is still considered borrowed when we try to use it. https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=193bdb3f0ffae931f060751c39ef9087 is a reduced test case.

The "proper" fix would be:

struct DocumentBuilder<'a, 'b> {
    u: &'a mut Unstructured<'b>,

… but that changes the signature of the type which would need to be updated at every use, including each impl