Cleanup and StackSlab

[Jake-Shadle]

• All unsafe usage now has a (sometimes minimal...) SAFETY: comment for my own sanity
• Changed some libc definitions to just use u8 instead of i8 (c_char) since they were annoying to deal with, cleaning up parts of the nic code
• Fixed undefined behavior in the netlink code that was doing the same unaligned pointer casting as the old packet code
• Simplified Packet by keeping it simple and removing the &'static mut [u8] and replacing it with a *,usize pair instead as most of the code is directly interacting with the pointer anways. Also fixed a bug where the capacity was incorrectly including the XDP_PACKET_HEADROOM
• Moved HeapSlab into a slab module, and added a new Slab trait and a new implementation, StackSlab which is a simple fixed size ring buffer
• Lots of internal cleanup