### Preliminary PRs:
- [x] #4597
- [x] #4599
- [x] #4600
- [x] #4602
- [x] #4603
- [x] #4604
- [x] #4605
- [x] #4607
- [x] #4627
- [x] #4629
### Quick overview over API/naming changes compared to `Lean.HashMap`
and `Batteries.HashMap`:
#### Lean
* `find?` -> `get?`/`getElem?`
* `find!` -> `get!`/`gtetElem!`
* `findD` -> `getD`
* `findEntry?` -> not implemented for now
* `insert'` -> `containsThenInsert` (order reversed in result)
* `insertIfNew` -> `getThenInsertIfNew?` (order reversed in result)
* `numBuckets` -> `Internal.numBuckets`
* `ofListWith` -> not implemented for now
* `Array.groupByKey` -> not implemented for now
* `merge` -> not implemented for now, but you can use `insertMany`
#### Batteries
* `modify` -> not implemented for now
* `mergeWith` -> not implemented for now
* `mergeWithM` -> not implemented for now
