Revert back to emulate ZipExtFile.seek

[althonos]

Using the lower level interface is:

• highly implementation-specific
• harder to understand
• generating bugs

This PR reverts back to a purely emulated seeking feature, i.e. seeking forward by reading.

[coveralls]

Coverage decreased (-0.0003%) to 99.951% when pulling 4fdc2e1 on althonos:fix-zipfs into 92e985c on PyFilesystem:master.

[willmcgugan]

Fair enough. Using the buffer was clever, but obviously complicated things considerably. I imagine the 99% use case would be to read the entire file sequentially anyway.

[althonos]

Yes, the ZIP format does not support random access anyway, so in the end the best practice would be either to read it sequentially or to decompress the archive.

[willmcgugan]

Thanks. I'll do a release shortly.