Allow deriving Pod for structs where generics are used only as PhantomData

[vadorovsky]

Currently, bytemuck disallows to derive Pod if the struct has any generics:

bytemuck/derive/src/traits.rs

Lines 55 to 57 in 8391afa

	Pod requires cannot be derived for non-packed types containing \
	generic parameters because the padding requirements can't be verified \
	for generic non-packed structs\

However, if generics are used only as PhantomData inside #[repr(C)] struct, that should be still OK. (I ended up doing unsafe impl bytemuck::Podmyself in such cases)

[wdanilo]

I believe this is duplicate of what I raised yesterday (at least both problems could be solved by the same thing): #190

[Lokathor]

I think a solution to 190 would solve this too but we can keep this issue open just in case.