AttesterSlashing number overflow

Impact

Possible consensus split given maliciously-crafted AttesterSlashing or ProposerSlashing being included on-chain.

Since we represent uint64 values as native javascript numbers, there is an issue when those variables with large (greater than 2^53) uint64 values are included on chain. In those cases, Lodestar may view valid AttesterSlashing or ProposerSlashing as invalid, due to rounding errors in large number values. This causes a consensus split, where Lodestar nodes are forked away from the main network.

Similarly Lodestar may consider invalid ProposerSlashing as valid, thus including in proposed blocks that will be considered invalid by the network.

Patches

https://github.com/ChainSafe/lodestar/pull/3977

Workarounds

Use BigInt to represent Slot and Epoch values in AttesterSlashing and ProposerSlashing objects. BigInt is too slow to be used in all Slot and Epoch cases, so we will carefully use BigInt just where necessary for consensus.