Improves numerical precision (if applicable) using pairwise summation:
https://en.wikipedia.org/wiki/Pairwise_summation

Thanks to Rasmus Larsen for the succinct binary tree aggregation pseudocode.

Also adds AsString for Variant types: this emits the Variant as a string via
its DebugString().

PiperOrigin-RevId: 340246073
Change-Id: I009281f46cbea30d6e33ecf79a1723d62e96cc6d