F.simple hashmap type

[floriankramer]

This pr turns the ad_utility::HashMap type into a typedef to avoid having to declare all required funtions of the hash map manually using a using statement.

It also replaces ad_utility::HashSet with a typedef of absl::flat_hash_set, switching away from google::dense_hash_set. I assume the same arguments for switching from googles dense_hash_map to abseil apply for hash sets, but in case they don't this change is it's own commit that can be easily removed from this pr.

[joka921]

Thank you, I think this heavily reduces confusion.