Prepare for LWIP challenge

[xlauko]

Challenge info:

Isolate Domain objects and operations

Isolate IPv4 domain - discover all structures and functions that uses IPv4 structure as arguments,
local variables, or fields.

Refine DSL

1. Define DSL for parsing received IPv4 packets.

2. Change IPv4 checksum to use crc16

3. Change IPv4 checksum to use crc32

4. Add means of tunneling by encapsulating IP over IP