Refactored obj.go to a more generic approach
Added object support for already implemented expressions
Added test for limit object
Small lint changes
Fixes https://github.com/google/nftables/issues/253
Before this commit: the unmarshaling of a verdict pointing
to a chain fails.
After this commit: the unmarshaling of a rule with a verdict
pointing to a chain succeeds and the information about the
chain gets put in `Verdict.Chain`.
Resolves: #105
Signed-off-by: Paul Greenberg <greenpau@outlook.com>