Buffers: add a `freeable` permission for `rfree` (with @nikswamy)
Buffers: add a `freeable` permission for `rfree` (with @nikswamy)

This fixes an unsoundness of Low*, which so far used to allow
`rfree`ing a sub-buffer of a `rcreate_mm`ed buffer.

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