| Elektra 0.11.0

Use Case: Index access to `KeySet`

# Summary

- Scope: `libelektra-core`
- Level: Developer Goal
- Actors: Core, Caller
- Brief: Caller accesses contents of `KeySet` by index

# Scenarios

- Precondition:
  - `KeySet` has been created.
  - `Key` has been inserted into `KeySet`.
- Main success scenario:
  - Caller requests `Key` at valid index (`0 <= i < size`) from `KeySet`
  - Core returns `Key *` for `Key` at index
- Alternative scenario: -
- Error scenario:
  - Caller requests `Key` at invalid index (`i < 0 || i >= size`) from `KeySet`
  - Core returns `NULL`
- Postcondition:
  - The returned `Key *` MUST be valid until the `Key` is removed from the `KeySet`.
- Non-functional Constraints: -