zodbdump: support --pretty option with a format to show pickles disassembly

Showing pickle disassembly can sometimes be useful to analyse details of the pickle content. We realized that in some data structures used in ERP5 the same string was saved multiple times in the same pickle and by using the exact same string (ie. for which s1 is s2 is True), the pickle will have the string only once and pickles are a bit smaller. For more reference, the context was https://lab.nexedi.com/nexedi/erp5/merge_requests/1560#note_154825

This introduces a new --pretty option that we will be able to extend later with more output formats.