stack-bound vs. travelling closures proposal.

[Brian Goetz]

This part of my note was wrong: we can allow
   unrestricted T->U  <:  restricted T->U

> Note that neither "restricted T->U" and "unrestricted T->U" are subclasses of
> each other; if either direction held, either you'd be able to pass a
> restricted closure to a unrestricted parameter (allows escape) or an
> unrestricted closure to a restricted parameter (safety violation.)