Transactional smart contracts

[mversic]

Make executing smart contract an instruction instead of part of Executable. I propose we introduce Execute<Wasm> and Execute<Trigger> instead of having ExecuteTrigger instruction

This will take care of some API weirdness because we'll now have Vec<InstructionBox> and will also enable us to combine smart contract execution with other instructions (or smart contracts) in a transaction where all have to succeed together or fail together