Transform GroupingMap[By] into GroupingGenericMap[By]

This is a huge but quite straightforward transformation:
- `GroupingMap[By]` becomes `GroupingGenericMap[By]` with a generic type `M`
- `K: Hash` becomes `M: Map<Key = K>`
- `HashMap<K, ...>` becomes `M` with a `M: Map<Value = ...>` bound
- Remove `Hash` and `HashMap`
- `use_std` becomes `use_alloc`

Author: Philippe-Cholet