PhantomData detection is brittle

[eira-fransham]

Currently it will generate false positives on cases like:

struct PhantomData;

#[derive(new)]
struct Foo { bar: PhantomData }

and false negatives on cases like:

use std::marker::PhantomData as PD;

#[derive(new)]
struct Foo { bar: PD }

[Lucretiel]

Unfortunately, this isn't possible to solve with a proc macro. Proc macros run before any useful type information is available, so the only thing they have to go on is the actual type names that are present in the definition of the decorated type.