actorlib/structuring: add reverse parameter to Unpack and Pack
flow/actor: add packetized parameter for Sink and Source
flow/actor: fix eop direction
add generic CRCEngine, CRC32, CRCInserter and CRCChecker

CRCEngine implements a generic and optimized CRC LFSR. It will enable generation of CRC generators and checkers.
CRC32 is an implementation of IEEE 802.3 CRC using the CRCEngine.
CRC32Inserter and CRC32Checker have been tested on an ethernet MAC.
genlib/fsm: make first fsm.act() the reset state, even when using after_*/before_* methods before fsm.act
Merge branch 'master' of https://github.com/m-labs/migen