The prohbition against `handle-evt' on `handle-evt' is as
document and as originally intended. I'm not sure why it
was allowed.
Existing programs that use `handle-evt' incorrectly
can break. I found and fixed one incorrect use and one
questionable use in the Racket tree (which is a small
minority of the uses of `handle-evt' in the tree).
* A long-standing bug, which happened with
(let ([x (lazy (delay 1))]) (force x) (force x))
not being properly handled.
* Added `delay/strict', mostly for the below.
* Made srfi/45 reprovide it as `eager'.
* Also restricted the exports from srfi/45 to its interface.
All of these issues were reported by Andreas Rottmann.
