RFC for "fat objects" for DSTs

This is intended as an alternative to #5.
"using the fat pointer is more flexible because you can always convert a thin pointer into a fat one, but not vice versa"

Flexibility is in the eye of the beholder here, no?  In particular: Taking the fat pointer provides flexibility for the clients of a method (because they will be able to call it regardless of which they are given), but it restricts the method itself: the method itself will not be able to convert the given fat pointer to a DST object into a thin pointer to a fat object.

I assume that *some* methods will need to work with the thin pointers (namely methods expected to construct new instances of types holding thin pointers to fat objects).  So the rule of thumb to always to take a fat pointer for borrowed pointer parameters will not always apply.