Strengthen precond of FStar.BufferNG.rcreate following FStar.Pointer.Base.ecreate
Strengthen precond of FStar.BufferNG.rcreate following FStar.Pointer.Base.ecreate

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