Implement missing APIs

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`](https://github.com/rust-lang/hashbrown/blob/master/src/map.rs) should get you quite far. Remember to also add the corresponding tests!

Compare src/map.rs with hashbrown/src/map.rs, focusing first on the unchecked RawEntry API and Drain iterators. Read the existing map tests and add corresponding coverage for each implemented API. Done means both unchecked checklist items are supported consistently with the reference implementation and their tests pass.

Written by the indexing model from the issue text.