Rename `BeginIteration`/`EndIteration` to `BeginBatching`/`EndBatching`
to reflect that the mechanism now covers both entity iteration and
system execution.
- Update `ISystem` and `ITickedSystem` to use `RunImpl` for logic.
- Add `SystemExtensions.Run` to wrap system execution in a batching
scope.
- Update `SystemGroup` to wrap system execution in a batching scope.
- Update `WorldQueryExtensions` to use the new batching terminology.