EIP-4895: Beacon chain push withdrawals as operations

[anvacaru]

Implementing withdrawal operations as specified in EIP-4895.

• adding a new <withdrawals> cell map subconfiguration to the <network> that is used to keep track of withdrawals.
  Similar to how <messages> is implemented, <withdrawals> has two additional Lists:
  • <withdrawalsPending> to keep track of the withdrawals that need to be applied.
  • <withdrawalsOrder> to preserve the order of the withdrawals during the check rules after the execution has finished.
• adding new EthereumCommands productions:
  • load "withdraw" that handles the rlp decoding of the withdrawals array. For each item, the following two productions will be applied:
  • mkWD - that creates a new <withdrawal> cell map item.
  • loadWithdraw - that initialises the newly created <withdrawal> with values that are provided from the rlp payload.
  • newWithdrawalID - function that is used to generate a new index**.
• updating .k specs

** newWithdrawalID shouldn't be necesary, as according to the specs:

Withdrawals provide key information from the consensus layer:
1. a monotonically increasing index, starting from 0, as a uint64 value that increments by 1 per withdrawal to uniquely identify each withdrawal
4. a nonzero amount of ether given in Gwei (1e9 wei) as a uint64 value.

So the provided withdrawal index should be enough to keep track of the withdrawals.
However, the conformance tests have multiple withdrawals with the 0 index and 0 amount (example).