IPv6 and robustness improvements

• Add ICMPv6 error reporting functions (#101)
• Add universal IP address converters (#108)
• Add error_message functions for human-readable errors (#98)
• Improve debug logging for ICMP Destination Unreachable packets.
• Filter incoming frames by MAC address to stop sending unnecessary RSTs. (#114)
• Unhook unused modules Sliding_window and Profiler from the build. (#112)