Implement missing APIs

[jonhoo]

For the initial prototyping, a number of APIs were left out. These should be added back in (ordered roughly in order of difficulty):

• [x] Entry API\
  • [x] OccupiedEntry::replace_*
• [x] serde and rayon support
• [ ] RawEntry API
• [ ] The Drain iterators
• [x] The reserve and shrink methods (and their siblings)
• [x] Support for the Extend trait
• [x] HashSet

In general, doing a diff between src/map.rs and hashbrown/src/map.rs should get you quite far. Remember to also add the corresponding tests!