Revert "should be reverted; working around the absence of subtyping so far"
Revert "should be reverted; working around the absence of subtyping so far"

This reverts commit 82bd68e2f9aa9e93e6fc55bd9b8bed3c1cb488d1.

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