Fix bugs in proposer calculation post-Fulu (#8101)

As identified by a researcher during the Fusaka security competition, we were computing the proposer index incorrectly in some places by computing without lookahead.


  - [x] Add "low level" checks to computation functions in `consensus/types` to ensure they error cleanly
- [x] Re-work the determination of proposer shuffling decision roots, which are now fork aware.
- [x] Re-work and simplify the beacon proposer cache to be fork-aware.
- [x] Optimise `with_proposer_cache` to use `OnceCell`.
- [x] All tests passing.
- [x] Resolve all remaining `FIXME(sproul)`s.
- [x] Unit tests for `ProtoBlock::proposer_shuffling_root_for_child_block`.
- [x] End-to-end regression test.
- [x] Test on pre-Fulu network.
- [x] Test on post-Fulu network.


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

testing/ef_tests/src/cases/fork.rs

@@ -60,7 +60,7 @@ impl<E: EthSpec> Case for ForkTest<E> {
     fn result(&self, _case_index: usize, fork_name: ForkName) -> Result<(), Error> {
         let mut result_state = self.pre.clone();
         let mut expected = Some(self.post.clone());
-        let spec = &E::default_spec();
+        let spec = &fork_name.make_genesis_spec(E::default_spec());
 
         let mut result = match fork_name {
             ForkName::Base => panic!("phase0 not supported"),