Zipper: Hole ↦ Top
Zipper: Path ↦ Context

This avoids having an alias and two names (from two metaphors) for the
same thing.
Zipper: Location/Subterm ↦ Subtree

Subtree is a single name which is appropriate both generically in the
context of Zipper and specifically inside our transformations.

This avoids the last confusing alias in our code.

I also tried to rename all occurrences of Subterm/Location in other
identifiers.