Rename MultiDict back to original MultiMap or SetMultiMap

[er1c]

I think it may make the most sense to rename MultiDict to SetMultiMap, and potentially make MultiMap into a generic trait.

1. It helps remove the @Deprecation warning if renamed back to MultiMap
2. "MultiMap" is a more universally used terminology for this type of collection. (e.g. Guava has ArrayListMultimap, ListMultimap, SetMultimap, and etc; C++ STL is std::multimap; Rust is MultiMap, Apache Commons is MultiMap, etc)
3. The existing name also does support or convey the collection type of the key's values. (e.g. it currently implements Set, but there is no way to extend it to use an Array, Vector or List instead)
4. The code still uses MultiMap in various locations (like in the hashcode!), and the CC[_] is MapFactory (e.g. not DictFactory)
5. There are no other Dictionary terms in any of the Scala collections. It's Map for any scala code, only the java converters have the mention of a dictionary. MultiDict isn't scala idiomatic.

[SethTisue]

duplicate of #17? cc @joshlemer

[er1c]

@SethTisue thanks, I missed that other issue, closing and moving comment to the other issue.