Remove PhantomData about scale-info

[ascjones]

I believe the idea is that any fields which are PhantomData are not even included in a type definition at all (since they have no encoded representation in SCALE). @Robbepop might elaborate more what he means.

[Robbepop]

Probably will need to update derive macro to skip over PhantomData fields.

Note that checks like these should always happen on a type level, never on a syntactical level so this will get a bit more involved in a proc. macro. You might be in need of additional helper traits to help in this process.

[dvdplm]

Do I understand this correctly? We want this:

scale-info/src/tests.rs

Lines 71 to 77 in 4188f02

	assert_type!(
	PhantomData<i32>,
	Type::builder()
	.path(Path::prelude("PhantomData"))
	.type_params(tuple_meta_type!(i32))
	.composite(Fields::unit())
	);

…to be this:

	assert_type!(
		i32,
		Type::builder()
			.path(Path::prelude("PhantomData"))
			.type_params(tuple_meta_type!(i32))
			.composite(Fields::unit())
	);

(and pass, naturally)?