This centralises the logic for initialising the decompressor and
decrypter when 1. opening the file and 2. seeking backwards in a file
stream.

This commit resolves an issue where seeking on an encrypted zipfile
would cause the crc keys to go out of sync resulting in data read after
the seek to be garbled.

This commit refactors `_ZipDecrypter` to be a class (`CRCZipDecrypter`)
that can be reinitialised when seeking back to the beginning of a zipped
file. In doing so, some of the logic relating to configuring the
decrypter which was found in `ZipFile` has been moved to the decrypter
class.