Return `Ipv4Packet` in Packet builder

[zhitaoli-6]

Hi, thank you for this open source project! It does help me to use pnet.

I have some comment for the following code:

lib_blaster/src/builder.rs

Line 111 in 38c72b3

/// Convert `Pkt` to a immutable `TcpPacket`

Ipv4Packet maybe more clear because we are working in Lay3 though TcpPacket can work here. I have test Ipv4Packet here and it does work.

BTW, could you tell me why total_length can be missing in the ip_header? It seems that someone replaces this field with the right value and re-calculate the CRC of ip header.

lib_blaster/src/builder.rs

Line 52 in 38c72b3

// Miss: total length