add getActions to LocalBlockchain

[Trivo25]

Description

This is a follow up PR to #323 that introduces getActions (fetchSequenceEvents) to theLocalBlockchain and the reducer on smart contracts.

In order to get it working #324 is required, since it adds the needed sequenceState field to zkapp accounts. (DONE)

Under the hood

Similar to fetchEvents, emitted actions are now stored in a variable called actions and the LocalBlockchain exposes a closure 'getActions` that returns all emitted actions.

zkApp instances can now fetch sequence events (actions) as follows:

this.reducer.getActions({
    fromActionHash?: Field;
    endActionHash?: Field;
}): Field[][]

Question: getActions currently is a part of the reducer. Should it stay there or become its own method on the SmartContract class, similar to fetchEvents?

The example src/examples/state_update_rollup.ts has been extended to support multiple action rollups

closes #284

[mitschabaude]

Question: getActions currently is a part of the reducer. Should it stay there or become its own method on the SmartContract class, similar to fetchEvents?

should stay part of reducer, that gives us much better typing than we have with events. I think we should change events as well at some point

[mitschabaude]

lgtm!