## Why this should be merged
Support for configurable `core/types.Block` with RLP encoding, including
interplay with `Body`.
## How this works
`Block` doesn't export most of its fields so relies on an internal type,
`extblock`, for RLP encoding. This type is modified to implement the
`rlp.Encoder` and `Decoder` methods as a point to inject hooks using
`rlp.Fields` (as in #120 for `Body`).
`Block` shares the same registered extra type as `Body`. Unlike
`Header`, which has its own field in a `Block`, the fields in `Body` are
promoted to be carried directly. This suggests that (at least for pure
data payloads) the modifications might be equivalent (and
`ava-labs/coreth` evidences this). Should different payloads be
absolutely required in the future, we can split the types—the
`RegisterExtras` signature is already too verbose though 😢.
## How this was tested
Explicit inclusion of a backwards-compatibility test for
`NOOPBlockBodyHooks` + implicit testing via the multiple upstream tests
in `block_test.go`. Re implicit testing: default behaviour is now to use
the noop hooks even when no registration is performed, but if we change
this then the tests in `block_test.go` can still be called as subtests
from a test that explicitly registers noops.
---------
Signed-off-by: Arran Schlosberg <519948+ARR4N@users.noreply.github.com>
Co-authored-by: Quentin McGaw <quentin.mcgaw@avalabs.org>
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