Group subsequent labels in rollback log

[mal]

Each label that's encountered adds to the rollback log even if no interaction takes place between it and the previous label. In some cases this can lead to the rollback log becoming saturated and useful entries being pushed out.

To counter this Tom proposed that subsequent checkpoints originating from label statements could potentially be merged, such that whether a single label statement or one hundred label statement are present between two interactions only a single log entry would be present.

(Paraphrasing a discussion, and logging as an issue as I don't currently have time to dedicate to it.)