[skipruntime] Type cleanups and improvements

[bennostein]

• Use opaque type instead of raw bigint for subscription IDs
• Use bigint instead of converting to/from string for watermarks, and introduce an opaque type there as well
• Unify treatment of collection diffs/updates: now the same type is used for the return value of getDiff and the callback argument in subscribe (which previously took the same three values, but destructured into separate arguments). I think this is an improvement on the Notifier function type alias I deleted in #362 in terms of clarity, but open to reverting if people prefer the previous setup.
• Some minor semantics-preserving changes, getting rid of some single-letter type variables and tweaking comment prose.