AttesterSlashing number overflow

Lodestar is a TypeScript implementation of the Ethereum Consensus specification. Prior to version 0.36.0, there is a possible consensus split given maliciously-crafted `AttesterSlashing` or `ProposerSlashing` being included on-chain. Because the developers represent `uint64` values as native javascript `number`s, 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. Version 0.36.0 contains a fix for this issue. As a workaround, 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 one may carefully use `BigInt` just where necessary for consensus.