Memory trackers need to report two figures for an object, so that bytes already charged when they were allocated aren't counted a second time toward the per-operation memory limit:

• full: all bytes the object accounts for. This is what the existing size methods return, and what stage-level trackers use for spilling decisions.
• unique: the part of those bytes not already charged at allocation. This is the figure the per-operation limit should use, so each byte counts once.

Today the size methods return a single number. This change adds a sibling to each size method used by memory tracking, returning both figures as a pair. The change covers the core data types, not the components that own them, which will be covered in following PRs. No code calls the new methods yet, and later changes will adopt them.