feat: parent.subaccount(name) -> AccountId

[mitinarseny]

This PR adds sub_account method to AccountIdRef exposing a convenient API for creating sub accounts instead of doing it manually every time

[frol]

Resolves #23

@race-of-sloths score 3

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Current status: executed

Reviewer	Score
@frol	3

Your contribution is much appreciated with a final score of 3!
You have received 30 Sloth points for this contribution

@frol received 25 Sloth Points for reviewing and scoring this pull request.

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