* fix for #58
* add fixes for both retransmit and fuzz failure found
* fix infinite challenge ack due to RST+SYN+FIN corrupt packet in SYNRCV state
* fuzz: previous recovery ack path led to infinite transmit
* fuzz: calculate CRCs when fuzzing to reduce search space to non-CRC error cases
* add local fuzz corpus to tests
* add more cases to fuzz corpus
* tcp: bugfix for seq not in snd/rcv.wnd error
* accept dupe ACKs for window updates and prevent decrement of UNA
* add fixes for incorrect tcp functioning
* remove Conn.Available* methods in favor of Conn.Free* methods due to ambiguous name