Transform client code to state passing style

[j-mueller]

The client code of each contract is going to be compiled to an executable, the contract runtime. The contract runtime should be a stateless component. Therefore we need to transform all actions in the WalletAPI monad into functions that take the current state and produce a new state plus some events (register triggers, create transactions, etc.)

[michaelpj]

Note: to do this in strictly monadic style we'd need to serialize a continuation back to the client, which we almost certainly don't want to do.

Instead, we probably want applicative style computations, so all the client-side effects we want can be done up front. We could use selective functors to improve this further.

[j-mueller]

I had a look at the selective functors paper and it seems to be a great fit for what we're trying to do.

[j-mueller]

Implemented in #1013