Re-design how Entry and Group are stored in a database - from nested structs to
flat HashMaps stored within the Database struct, with cross-references by
EntryId and GroupId, for faster traversal.

Invariants in how the objects can be accessed ensure that the database is always
in a valid state, e.g. EntryId and GroupId refer to entries that still exist in
the database.

BREAKING CHANGE: Entry and Group access completely changed