Operations over Binary random access lists

[mmontin]

This PR proposes an implementations for the primitives on binary random access lists. It includes the following changes:

• addition of lookup and update for trees
• addition of an indexed version of bral to allow for recursive functions over the structure
• implementation of the actual primitives A question: what is the semantics of the remaining primitives, namely snoc, rear and front. My guess:
• snoc adds a tree of size k+1 at the end of the structure
• rear returns the last tree of the structure
• front returns the first tree of the structure

[mmontin]

Thanks for these comments @pedagand Do I have the right interpretation for the primitives I mentioned, namely front, rear and snoc ?

[pedagand]

I think so, yes.

[mmontin]

Thanks, I will fill these blanks and come up with less nasty implementations.