Reorganize containers (like hashmap, mutable_map, etc) into a dedicated collections folder

[jialunzhang-psu]

Motivations

• The list of packages in core is getting lengthy.
• After putting different containers into a separate collections folder, it becomes more convenient to share implementation between containers. For example, hashmap and hashset share the same underlying Robin Hood hashing. mutable_set and mutable_map can share the red-black tree implementation. Maintaining just one copy of the implementation reduces redundant code and tests.

Concern

• Suppose we have hashmap, hashset and robinhood_hash, three packages under the collections folder, while hashmap and hashset rely on robinhood_hash. How to make robinhood_hash invisible to the outside?

A potential PR is #324.