Remove pub from various fields in exported types, notably in extrinsic::extra

[jsdw]

Various types, notably in crate::extrinsic::extra, expose their inner workings (including PhantomData) to the world, eg:

pub struct ChargeAssetTxPayment<T: Config> {
    /// The tip for the block author.
    #[codec(compact)]
    pub tip: u128,
    /// The asset with which to pay the tip.
    pub asset_id: Option<u32>,
    /// Marker for unused type parameter.
    pub marker: PhantomData<T>,
}

and

pub struct ChargeTransactionPayment<T: Config>(
    #[codec(compact)] u128,
    pub PhantomData<T>,
);

And others. We should have a pass over the types that we export, and in cases like the above we should provide a better interface for creating/accessing fields that hides details like PhantomData.

As it stands, I'm not sure that most of the types in extra are ever even instantiated and aren't just used at the type level, but I'm not entirely clear at the mo?

[jsdw]

This is no longer relevant after #490, which reworks this stuff.