Support variable length structs as array elements

[fsareshwala]

I was trying to define a set of emboss structs from the batch scanning section of Android's Bluetooth vendor extensions. In the full version of the results that the Controller returns, there are some variable length array elements that are currently impossible to add to Emboss. I would have liked to do something like this:

struct LEBatchScanTruncatedResult:
  0     [+hci.BdAddr.$size_in_bytes]  hci.BdAddr         peer_address
  $next [+1]                          hci.LEAddressType  peer_address_type
  $next [+1]                          Int                tx_power
  $next [+1]                          Int                rssi
  $next [+2]                          UInt               timestamp

struct LEBatchScanFullResult:
  0     [+hci.BdAddr.$size_in_bytes]  hci.BdAddr         peer_address
  $next [+1]                          hci.LEAddressType  peer_address_type
  $next [+1]                          Int                tx_power
  $next [+1]                          Int                rssi
  $next [+2]                          UInt               timestamp
  $next [+1]                          UInt               advertising_data_length (ad_len)
  $next [+ad_len]                     UInt:8[ad_len]     advertising_data
  $next [+1]                          UInt               scan_response_length (sr_len)
  $next [+sr_len]                     UInt:8[sr_len]     scan_response_data

struct LEBatchScanReadResultsCommandCompleteEvent:
  let hdr_size = hci.CommandCompleteEvent.$size_in_bytes
  0     [+hdr_size]                                     hci.CommandCompleteEvent                 command_complete
  $next [+1]                                            hci.StatusCode                           status
  $next [+1]                                            BatchScanSubOpcode                       sub_opcode
    [requires: this == BatchScanSubOpcode.READ_RESULT_PARAMETERS]

  $next [+1]                                            BatchScanReadMode                        read_mode
  $next [+1]                                            UInt                                     num_records
  if read_mode == BatchScanReadMode.TRUNCATED_MODE:
    $next [+LEBatchScanTruncatedResult.$size_in_bytes]  LEBatchScanTruncatedResult[num_records]  truncated_results

  if read_mode == BatchScanReadMode.FULL_MODE:
    let remaining = command_complete.return_parameters_size-hdr_size-4
    $next [+remaining]                                  LEBatchScanFullResult[num_records]       full_results

Unfortunately, this results in an error from Emboss: "error: Array elements must be fixed size." We should figure out a way to add such functionality in so that users don't have to awkwardly break up packets into multiple structs.