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>

beacon_node/beacon_chain/Cargo.toml

@@ -9,9 +9,9 @@ default = ["participation_metrics"]
 write_ssz_files = []  # Writes debugging .ssz files to /tmp during block processing.
 participation_metrics = []  # Exposes validator participation metrics to Prometheus.
 test_logger = [] # Print log output to stderr when running tests instead of dropping it
+fork_from_env = [] # Initialise the harness chain spec from the FORK_NAME env variable
 
 [dev-dependencies]
-int_to_bytes = { path = "../../consensus/int_to_bytes" }
 maplit = "1.0.2"
 environment = { path = "../../lighthouse/environment" }
 
@@ -45,6 +45,7 @@ eth1 = { path = "../eth1" }
 futures = "0.3.7"
 genesis = { path = "../genesis" }
 integer-sqrt = "0.1.5"
+int_to_bytes = { path = "../../consensus/int_to_bytes" }
 rand = "0.7.3"
 rand_core = "0.6.2"
 proto_array = { path = "../../consensus/proto_array" }
@@ -56,7 +57,7 @@ safe_arith = { path = "../../consensus/safe_arith" }
 fork_choice = { path = "../../consensus/fork_choice" }
 task_executor = { path = "../../common/task_executor" }
 derivative = "2.1.1"
-itertools = "0.9.0"
+itertools = "0.10.0"
 regex = "1.3.9"
 exit-future = "0.2.0"
 slasher = { path = "../../slasher" }