Make Relay edge cursors easily readable using wrapper objects.

Since relayStylePagination edges could be Reference objects, we either
need to read their properties using options.readField (like we do for
edge.node in the read function), or keep track of those properties
separately in the internal representation of the field.

This second strategy turns out to be more convenient because the merge
function sometimes needs to update the cursor associated with an edge
using information from pageInfo. Modifying other parts of the cache while
you're merging a field can be awkward, so it's simpler and cleaner to
update the cursor property of a wrapper object.