Add allowCalls cheatcode

[palinatolmach]

Related: runtimeverification/kontrol#926.

This PR adds the allowCalls cheatcode that enables whitelisting address and specific calldata:

    // Adds an address and calldata to the whitelist.
    function allowCalls(address, bytes calldata) external;

constituting a more specific version of

    // Adds an address to the whitelist.
    function allowCallsToAddress(address) external;

It can then be used as follows:

  function testExecuteOnlyCallsSubmittedProposals(uint256 proposalId) external {
      Timelock timelock = new Timelock();
      
      // Whitelist one call to the timelock
      bytes memory timelockCalldata = abi.encodeWithSelector(
	      Timelock.execute.selector,
	      proposalId
      );
      kevm.allowCalls(address(timelock), timelockCalldata);

      // Whitelist another call to the timelock
      bytes memory timelockCalldataAnother = abi.encodeWithSelector(
	      Timelock.anotherCall.selector,
	      proposalId
      );
      kevm.allowCalls(address(timelock), timelockCalldataAnother);
      
      // Perform a whitelisted call
      timelock.execute(proposalId);

      // Perform another non-whitelisted call that will cause failure
      timelock.set(proposalId);
  }