Changes made:
* Reader.flg is not used anywhere else other than readHeader and
does not need to be stored.
* Store Reader.digest and Writer.digest as uint32s rather than as
a hash.Hash32 and use the crc32.Update function instead. This simplifies
initialization logic since the zero value of uint32 is the initial
CRC-32 value. There are no performance detriments to doing this since
the hash.Hash32 returned by crc32 simply calls crc32.Update as well.
* s/[0:/[:/ Consistently use shorter notation for slicing.
* s/RFC1952/RFC 1952/ Consistently use RFC notation.