Reinstate the SigningProcess API

[sjoerdvisscher]

Fixes #373

I thought the SigningProcess code was old and should be removed. But after reading the "Control effects" note in MultiAgent.hs it actually made sense to me to have this, to emulate "outside the contract" signing processes, like emailing the transaction around for signatures.

So fixed it to let it have an effect again, and also fixed that it workes with the cardano-ledger validation.

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