Improvements to MerkleList and Iterator

[Trivo25]

• startIteratingFromOldest() to add an iterator which iterates from oldest to newest elements (n_i to n_0), helpful for reducer-like applications that need to iterate through historical data
• add MerkleList.fromReverse creates a MerkleList from an array but reverses it internally
• demonstrates how to use an iterator for actions and reducers that reduce historical data

[mitschabaude]

I think you also have to replace next() with previous() and startIterating() with startIteratingFromLast() in all of our library code

[mitschabaude]

is this cloning necessary 🤔

[Trivo25]

yea, otherwise pop would literally steal the elements form the iterator :D I could have also just initiated a second array, but I thought I would clone it to make sure it actually does clone