It's not only helpful (to easily check where Serai's view of the external
network is) but it's necessary in case of a non-trivial chain fork to determine
which blockchain Serai considers canonical.
We prior required they had the same encoding, yet this ensures they do by
making them one and the same. This does require an large, ugly, From/TryInto
block which is deemed preferable for moving this more and more into syntax
(from semantics).
Further improvements (notably re: Extra) is possible, and this already lets us
strip some members from the Call enum.
Uses a full-fledged serai-abi to do so.
Removes use of UncheckedExtrinsic as a pointlessly (for us) length-prefixed
block with a more complicated signing algorithm than advantageous.
In the future, we should considering consolidating the various primitives
crates. I'm not convinced we benefit from one primitives crate per pallet.
Call and Event are both from the pallets, which are AGPL licensed. Accordingly,
they make serai-client AGPL licensed when serai-client must end up MIT
licensed. This creates a MIT-licensed variant of Calls and Events such that
they can be used by serai-client, enabling transitioning it to MIT.
Relevant to https://github.com/serai-dex/serai/issues/337.
