Add ways to access the `Bytes` version of an `ABI`

[aaroncox]

The setcode action on the system contract requires you pass in Bytes. If you have the ABI, you need to manually serialize it to get the bytes version (code below). The ABI class should have some method/getter that returns these values instead of requiring this boilerplate code.

const abi = ABI.from({
    version: 'eosio::abi/1.0',
    types: [],
    variants: [],
    structs: [],
    actions: [],
    tables: [],
    ricardian_clauses: [],
})

const result = Serializer.encode({
    object: abi,
    type: ABI,
})

console.log(result) // Bytes object
console.log(result.array) // Uint8Array
console.log(String(result)) // hex encoded string