- Renamed `BiMap` to `IdNameMap` to better reflect its purpose
- Extracted type parameters out of `IdNameMap` for ease of reuse
- Added `IdNameMap::insert` for convenience
- `UsersCache` now uses a single `RefCell` for both forward and
  backward maps; does not seem to break anything
- Refactored methods that use `UsersCache::{users,groups}`
  accordingly
- `UsersCache::default` is now implemented with `derive`; `HashMap`
  nonsense moved into the new `IdNameMap::default`

- Added `AllUsers` trait that provides `all_users()`
- Added `UsersSnapshot`
  - user data is pre-loaded, not fetched lazily
  - implements `AllUsers` and `Users`
- `MockUsers` now implements `AllUsers` exposing the list of added
  users
- Equivalent group functionality NYI
- Documentation not yet updated

- Added `AllGroups` trait that provides `all_groups()`
- `UsersSnapshot` now implements `Groups` and `AllGroups`
- `MockUsers` now implements `AllGroups`
- `UsersSnapshot::filtered` not yet updated: adds all groups
- Documentation not yet updated

`UsersSnapshot::filtered` has been split into two new methods to
accomodate the two probable use cases:
- `::filtered` now accepts a separate filter for groups
- `::only_users` only accepts groups that are some included user's
  primary group.

- Documentation of `UsersCache` was moved from module to the struct
  itself to keep the module doc from bloating to a billion lines.
- Description of `UsersCache` in crate doc shortened slightly as it
  is no longer the only solution.

Signed-off-by: Sandro-Alessio Gierens <sandro@gierens.de>