[chainstate] Add support for pay-to-contract in coinbase transaction variant

[jcnelson]

Make it so a coinbase transaction can pay out to a smart contract address, instead of just a standard address.

[jcnelson]

This will go further: make it possible to supply an alternative recipient address (contract or standard) in the coinbase tx. Then, miners can pay their block rewards to cold storage.

[blockstack-devops]

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