Remove `bigint` wrapper type

[ndzik]

• Removes the bigint wrapper type BigInteger.
• Adds a jsonBigIntMember decorator for use with bigints and TypedJSON
• ABIEncoding didn't have to be touched, since we never relied on the AbiEncodable implementation of BigInteger and manually passed them as ["uint256", ourBigInteger] or ["uint64", ourBigInteger]. Every ourBigInteger is now of type bigint, which can be natively encoded by ethers.utils.defaultAbiCoder. We previously casted our BigInteger like this:

function asABI() {
  return ethers.BigNumber.from(this.value); // BigInteger -> bigint -> BigNumber
}

The encoding of BigNumber and bigint by ethers.utils.defaultAbiCoder are equivalent, so that is the reason why we only needed to change the bigint encoding for JSON.