Remove Index type from Config trait

[tadeohepperle]

fixes #1061

Whereever we previously required a Config::Index, now any type that implements Into<u64> can be used.

[jsdw]

I'd keep this simple and just have it take a concrete u64; it's only called internally anyway, and so let's avoid extra generics unless they make the API more ergonomic for users :)

[jsdw]

Looks good to me! I'd just remove the impl Into<u64> generics (it's easy for a user to provide a u64 off the bat; I don't see the point of making it generic really :))

Did you double check that the signed extension itself always wants a u64? That's the one wrinkle that might complicate things a touch!

[jsdw]

Ah, a wrinkle! I had a look at the CheckNonce signed extension impl:

pub struct CheckNonce<T: Config>(#[codec(compact)] pub T::Index);

impl<T: Config> CheckNonce<T> {
	/// utility constructor. Used only in client/factory code.
	pub fn from(nonce: T::Index) -> Self {
		Self(nonce)
	}
}

Ie the nonce type is whatever the Index type is, and isn't necessarily a u64!

This means that we would indeed have to use the metadata to find out the size of the Index type in the CheckNonce signed extension in order to actually remove this param (which I think is what you originally were looking to do, so apologies for the misdirection there!!)

[jsdw]

Scrap that; @tadeohepperle pointed out that the nonce type is compact encoded in the signed extension, and so it's irrelevant what numeric type it actually is, so we're still good :)

[jsdw]

Nit: no more u64 type needed now :)

[jsdw]

Looks great!