Provide support for private Aliases

It would be helpful to support private/local Aliases that are not exposed in the generated code. That way they can be used for computing offsets and sizes, but without polluting the fields shown in the generated code.

For example, something like the following would allow using `hdr_size` in the struct, but wouldn't add `hdr_size` to the generated code.

```
private let hdr_size = EventHeader.$size_in_bytes
0 [+hdr_size] EventHeader header
$next [+1] UInt num_hci_command_packets
$next [+2] OpCode command_opcode_enum
$next-2 [+2] OpCodeBits command_opcode_bits
```