Revert "Upgrade TaggedUnion, simplify gread_tag, gfield"
Revert "Upgrade TaggedUnion, simplify gread_tag, gfield"

This reverts commit 2d85017b0f5a228121237a397625a1b5b9534861.

A-Manning/FStar,A-Manning/FStar,A-Manning/FStar,FStarLang/FStar,FStarLang/FStar,FStarLang/FStar