Remove public access to `AdditiveShare` members #3475

Job	Run time
Additional Builds and Concurrency Tests	13m 59s
Basic Checks	11m 14s
Measure coverage	11m 42s
Release builds and tests	7m 54s
sanitize (address)	4m 58s
sanitize (leak)	3m 31s
	53m 18s

Provide feedback