Serialize pagination cursors for external consumers

[theronic]

This would just be a convenience enhancement because for external consumers, it's easier to receive / send strings over the wire, esp. when they don't care about the conetnts. For now, can use pr-str / EDN read-string.

Can be as simple as ":" e.g. "server:123"

base/Cursor is currently a map of {:keys [path-index resource-id]}. Internally could be {:keys [:path :id]} and stringify in the Spice impl.