pallet macro: easier syntax for #[pallet::pallet] with struct Pallet<T>(_)

[gui1117]

This PR makes the declaration of pallet placeholder less boilerplatey.

now user can just write:

#[pallet::pallet]
pub struct Pallet<T>(_);

And the macro will replace _ with the expected PhantomData.

Note that this macro doesn't forbid to write it explicitly like:

#[pallet::pallet]
pub struct Pallet<T>(sp_std::maker::PhantomData<T>);

If this PR gets into before substrate 3.0 I guess we can forbid any other syntax than _. otherwise I guess better to let user choose.