Tree hash caching and optimisations for Altair (#2459)

## Proposed Changes

Remove the remaining Altair `FIXME`s from consensus land.

1. Implement tree hash caching for the participation lists. This required some light type manipulation, including removing the `TreeHash` bound from `CachedTreeHash` which was purely descriptive.
2. Plumb the proposer index through Altair attestation processing, to avoid calculating it for _every_ attestation (potentially 128ms on large networks). This duplicates some work from #2431, but with the aim of getting it in sooner, particularly for the Altair devnets.
3. Removes two FIXMEs related to `superstruct` and cloning, which are unlikely to be particularly detrimental and will be tracked here instead: https://github.com/sigp/superstruct/issues/5

beacon_node/http_api/src/lib.rs

@@ -939,7 +939,6 @@ pub fn serve<T: BeaconChainTypes>(
             blocking_json_task(move || {
                 block_id
                     .block(&chain)
-                    // FIXME(altair): could avoid clone with by-value accessor
                     .map(|block| block.message().body().attestations().clone())
                     .map(api_types::GenericResponse::from)
             })