Allow explicit call ordering within a pallet

[joepetrowski]

Similar to explicit ordering in the construct_runtime macro, we could annotate calls within a pallet with an index. This would have a few advantages, namely the ability to remove a function without affecting the indices of other calls.

This could be useful in pallet versioning and transaction version, where a pallet's version would need to increment when there is a breaking change to these indices/function signatures. By leaving call indices empty on function removal, wallets/signers don't need to worry about signing an unintended call as it would be null.

Transaction version could either increment on pallet version changes or be reserved for changes in the serialization (e.g. signed extensions).

[apopiak]

IMO the transaction_version would still need to change on removal, but the migration would be trivial.

[bkchr]

IMO the transaction_version would still need to change on removal, but the migration would be trivial.

No, because there would be no harm. The call would fail to decode, aka nothing serious would happen.

[KiChjang]

Was trying to implement this lately, but found out that enum discriminants on non-unit variants are experimental. We'll need to figure out another mechanism for associating an index with every call in the enum, and use that to encode/decode correctly.

[xlc]

scale codec does support macro attributes to specific encode index

[bkchr]

Yeah and we use exactly this in construct_runtime! for pallets.