So far, `withState` and `withBlck` templates could only be used to have
convenience access to fork-agnostic BeaconState and BeaconBlock fields.
This patch:
- injects an additional `stateFork` constant that allows to use
`when` expressions to also access Altair and Merge-specific fields.
- introduces a `withStateAndBlck` template to support operating on both
a `BeaconState` and `BeaconBlock` at a time.
- makes sync committee related functions Merge aware.
- changes a couple if-else trees for forks into case statements so that
forgotten future forks are promoted to compile-time errors.
9ee134324b