bug#21899: let/ec continuations not distinct under compiler

[Zefram]

One more variant:

scheme@(guile-user)> (let ((x (list 'a))) (eq? x (list 'a)))
$1 = #t
scheme@(guile-user)> ,opt (let ((x (list 'a))) (eq? x (list 'a)))
$2 = (let ((x (list 'a))) (eq? x x))

-zefram