## Why this should be merged
There are 3 places at which we perform the same, sensitive logic to
access registered payloads and as we modify more types this is likely to
expand. (e.g. `types.Header`).
## How this works
Introduces `pseudo.Accessor` to abstract the reusable code.
## How this was tested
Existing unit tests. Note that the `types.StateAccount` tests needed a
minor refactor to provide the assertions with access to the
`ExtraPayloads[T]` without introducing generic types anywhere.
## Why this should be merged
Provides access to extra payloads registered on `types.SlimAccount`, not
just on regular `types.StateAccount`. This is a breaking syntax change
to have the method reflect the behaviour.
## How this works
Modify existing method to accept the `ExtraPayloadCarrier` interface
instead of `*StateAccount`. The interface is implemented by both
`*StateAccount` and `*SlimAccount`.
## How this was tested
Covered by existing testing.
## Why this should be merged
Fixes invariant broken by introduction of `types.StateAccountExtra`.
## How this works
`state.stateObject.empty()` now also requires that the underlying
account's `Extra` field carries the zero value if a type is registered,
or is itself nil.
## How this was tested
New unit test.
---------
Co-authored-by: Darioush Jalali <darioush.jalali@avalabs.org>
