GetCallIndex trait (#13558)

* GetCallIndex trait

* final impl

* ".git/.scripts/commands/fmt/fmt.sh"

* Docs

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* One more test

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Doc

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

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Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: command-bot <>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>