eeprom24xx: avoid access to caller's data after validity ends

eeprom24xx: avoid access to caller's data after validity ends

The at24 EEPROM decoder's previous implementation happened to access
caller's data even after the .decode() method invocation ended, and
their content has changed or the data was not valid any longer.

Get deep copies for those details which broke the test suite. Prepare
"generous" deep copies for other data which currently doesn't trigger
exceptions, but might be waiting for an accident to happen. Careful
inspection of the complex implementation and relaxing the current greed
of this commit remains for future commits. Comment heavily for awareness.

It is assumed that the 'databyte' name is misleading. And that much of
this upper layer decoder's complexity would be obsoleted by the lower
layer decoder's providing more useful packets (bytes and their ACK state,
read/write phases of transfers, complete transfers up to STOP, etc).
This commit does not address those readability or layering concerns.