Use SmallVec in Bitfield (#3025)

## Issue Addressed

Alternative to #2935

## Proposed Changes

Replace the `Vec<u8>` inside `Bitfield` with a `SmallVec<[u8; 32>`. This eliminates heap allocations for attestation bitfields until we reach 500K validators, at which point we can consider increasing `SMALLVEC_LEN` to 40 or 48.

While running Lighthouse under `heaptrack` I found that SSZ encoding and decoding of bitfields corresponded to 22% of all allocations by count. I've confirmed that with this change applied those allocations disappear entirely.

## Additional Info

We can win another 8 bytes of space by using `smallvec`'s [`union` feature](https://docs.rs/smallvec/1.8.0/smallvec/#union), although I might leave that for a future PR because I don't know how experimental that feature is and whether it uses some spicy `unsafe` blocks.

consensus/types/Cargo.toml

@@ -45,6 +45,7 @@ parking_lot = "0.11.1"
 itertools = "0.10.0"
 superstruct = "0.4.0"
 serde_json = "1.0.74"
+smallvec = "1.8.0"
 
 [dev-dependencies]
 criterion = "0.3.3"

consensus/types/src/attestation.rs

@@ -110,9 +110,34 @@ impl<T: EthSpec> SlotData for Attestation<T> {
 
 #[cfg(test)]
 mod tests {
+    use super::*;
     use crate::*;
 
-    use super::*;
+    // Check the in-memory size of an `Attestation`, which is useful for reasoning about memory
+    // and preventing regressions.
+    //
+    // This test will only pass with `blst`, if we run these tests with Milagro or another
+    // BLS library in future we will have to make it generic.
+    #[test]
+    fn size_of() {
+        use std::mem::size_of;
+
+        let aggregation_bits =
+            size_of::<BitList<<MainnetEthSpec as EthSpec>::MaxValidatorsPerCommittee>>();
+        let attestation_data = size_of::<AttestationData>();
+        let signature = size_of::<AggregateSignature>();
+
+        assert_eq!(aggregation_bits, 56);
+        assert_eq!(attestation_data, 128);
+        assert_eq!(signature, 288 + 16);
+
+        let attestation_expected = aggregation_bits + attestation_data + signature;
+        assert_eq!(attestation_expected, 488);
+        assert_eq!(
+            size_of::<Attestation<MainnetEthSpec>>(),
+            attestation_expected
+        );
+    }
 
     ssz_and_tree_hash_tests!(Attestation<MainnetEthSpec>);
 }

consensus/types/src/test_utils/test_random/bitfield.rs

@@ -1,9 +1,10 @@
 use super::*;
 use crate::{BitList, BitVector, Unsigned};
+use smallvec::smallvec;
 
 impl<N: Unsigned + Clone> TestRandom for BitList<N> {
     fn random_for_test(rng: &mut impl RngCore) -> Self {
-        let mut raw_bytes = vec![0; std::cmp::max(1, (N::to_usize() + 7) / 8)];
+        let mut raw_bytes = smallvec![0; std::cmp::max(1, (N::to_usize() + 7) / 8)];
         rng.fill_bytes(&mut raw_bytes);
         Self::from_bytes(raw_bytes).expect("we generate a valid BitList")
     }
@@ -11,7 +12,7 @@ impl<N: Unsigned + Clone> TestRandom for BitList<N> {
 
 impl<N: Unsigned + Clone> TestRandom for BitVector<N> {
     fn random_for_test(rng: &mut impl RngCore) -> Self {
-        let mut raw_bytes = vec![0; std::cmp::max(1, (N::to_usize() + 7) / 8)];
+        let mut raw_bytes = smallvec![0; std::cmp::max(1, (N::to_usize() + 7) / 8)];
         rng.fill_bytes(&mut raw_bytes);
         Self::from_bytes(raw_bytes).expect("we generate a valid BitVector")
     }