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):

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