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>

consensus/state_processing/src/per_block_processing/errors.rs

@@ -11,6 +11,8 @@ use types::*;
 /// (e.g., when processing attestations instead of when processing deposits).
 #[derive(Debug, PartialEq, Clone)]
 pub enum BlockProcessingError {
+    /// Logic error indicating that the wrong state type was provided.
+    IncorrectStateType,
     RandaoSignatureInvalid,
     BulkSignatureVerificationFailed,
     StateRootMismatch,
@@ -45,11 +47,16 @@ pub enum BlockProcessingError {
         index: usize,
         reason: ExitInvalid,
     },
+    SyncAggregateInvalid {
+        reason: SyncAggregateInvalid,
+    },
     BeaconStateError(BeaconStateError),
     SignatureSetError(SignatureSetError),
     SszTypesError(ssz_types::Error),
     MerkleTreeError(MerkleTreeError),
     ArithError(ArithError),
+    InconsistentBlockFork(InconsistentFork),
+    InconsistentStateFork(InconsistentFork),
 }
 
 impl From<BeaconStateError> for BlockProcessingError {
@@ -76,6 +83,12 @@ impl From<ArithError> for BlockProcessingError {
     }
 }
 
+impl From<SyncAggregateInvalid> for BlockProcessingError {
+    fn from(reason: SyncAggregateInvalid) -> Self {
+        BlockProcessingError::SyncAggregateInvalid { reason }
+    }
+}
+
 impl From<BlockOperationError<HeaderInvalid>> for BlockProcessingError {
     fn from(e: BlockOperationError<HeaderInvalid>) -> BlockProcessingError {
         match e {
@@ -339,3 +352,11 @@ pub enum ExitInvalid {
     /// been invalid or an internal error occurred.
     SignatureSetError(SignatureSetError),
 }
+
+#[derive(Debug, PartialEq, Clone)]
+pub enum SyncAggregateInvalid {
+    /// One or more of the aggregate public keys is invalid.
+    PubkeyInvalid,
+    /// The signature is invalid.
+    SignatureInvalid,
+}