Objects implementation refactor

[turekt]

Hi,

as per our discussion in #253, I have implemented a change for a more generic implementation of nftables objects. In addition to the refactor I have added a test to demonstrate that already implemented expressions are now supported as objects and made a few lint changes. Note that some of the expressions are currently not implemented in the lib (i.e. synproxy, ct_helper, ct_expect, ...). These can now be easily added to the expr package to offer support. I'll see to adding them after we agree on this refactor change.

Let me know what you think.