Altair consensus changes and refactors (#2279)

## Proposed Changes

Implement the consensus changes necessary for the upcoming Altair hard fork.

## Additional Info

This is quite a heavy refactor, with pivotal types like the `BeaconState` and `BeaconBlock` changing from structs to enums. This ripples through the whole codebase with field accesses changing to methods, e.g. `state.slot` => `state.slot()`.


Co-authored-by: realbigsean <seananderson33@gmail.com>

boot_node/src/lib.rs

@@ -51,7 +51,6 @@ pub fn run(matches: &ArgMatches<'_>, eth_spec_id: EthSpecId, debug_level: String
     if let Err(e) = match eth_spec_id {
         EthSpecId::Minimal => main::<types::MinimalEthSpec>(matches, log),
         EthSpecId::Mainnet => main::<types::MainnetEthSpec>(matches, log),
-        EthSpecId::V012Legacy => main::<types::V012LegacyEthSpec>(matches, log),
     } {
         slog::crit!(slog_scope::logger(), "{}", e);
     }