[Altair] Sync committee pools (#2321)

Add pools supporting sync committees:
- naive sync aggregation pool
- observed sync contributions pool
- observed sync contributors pool
- observed sync aggregators pool

Add SSZ types and tests related to sync committee signatures.

Co-authored-by: Michael Sproul <michael@sigmaprime.io>
Co-authored-by: realbigsean <seananderson33@gmail.com>

consensus/ssz/src/encode/impls.rs

@@ -2,6 +2,7 @@ use super::*;
 use core::num::NonZeroUsize;
 use ethereum_types::{H256, U128, U256};
 use smallvec::SmallVec;
+use std::sync::Arc;
 
 macro_rules! impl_encodable_for_uint {
     ($type: ident, $bit_size: expr) => {
@@ -231,6 +232,24 @@ impl<T: Encode> Encode for Option<T> {
     }
 }
 
+impl<T: Encode> Encode for Arc<T> {
+    fn is_ssz_fixed_len() -> bool {
+        T::is_ssz_fixed_len()
+    }
+
+    fn ssz_fixed_len() -> usize {
+        T::ssz_fixed_len()
+    }
+
+    fn ssz_append(&self, buf: &mut Vec<u8>) {
+        self.as_ref().ssz_append(buf)
+    }
+
+    fn ssz_bytes_len(&self) -> usize {
+        self.as_ref().ssz_bytes_len()
+    }
+}
+
 macro_rules! impl_for_vec {
     ($type: ty) => {
         impl<T: Encode> Encode for $type {