Enables traversing and modifying an AExpr tree.
Initialising an AEZipper with an AExpr-like type will work, however traversal and modification
functions work only on AExpr trees.
Traversal and modification functions may return a Snag in case of incorrect traversal (e.g. if one tries to zip down
when there is nothing to go down, or when one goes beyond 'end' of the tree)
Snags are carried forward, i.e. if a zipping function is passed a Snag it returns a Snag.

Why is there no library function to unwrap from an AEZipper?
The AEZipper adds context to the AExpr type that allows the AExpr tree to be 'traversed' and modified like
imperative programming languages would allow.
This context supports the possibility of failure to traverse which may occur if one tries to traverse in a 
direction that cannot be traversed from the current point of focus. The failure is represented by a Snag type.
The result of a succesful traversal is represented by a Zipper type that is essentially a tuple that comprises
of the part of the AExpr tree that is in focus, and the rest of the tree. The Snag and Zipper types are not
inherent to the AExpr trees themselves and thus once added they cannot be removed empirically.

Crumb 

A Crumb is a wrapper around an AExpr with some additional context.
The context is not determined by the AExpr that the Crumb wraps around but by another AExpr.
Hence it cannot be considered amenable to implementing as an applicative functor.