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

[Zefram <zefram@fysh.org>]

With guile 2.0.11:

scheme@(guile-user)> (use-modules (ice-9 control))
scheme@(guile-user)> (list 'a (let/ec ae (list 'b (let/ec be (be 2)))))
$1 = (a (b 2))
scheme@(guile-user)> (list 'a (let/ec ae (list 'b (let/ec be (ae 2)))))
$2 = (a (b 2))
scheme@(guile-user)> (list 'a (let/ec ae (list 'b (ae 2))))
$3 = (a 2)

The middle of these three cases is wrong: it attempts to invoke the outer
escape continuation, but only goes as far as the target of the inner one,
which it isn't using.  It therefore produces the same result as the first
case, which invokes the inner escape continuation.  It ought to behave
like the third case, which shows that the outer escape continuation can
be successfully invoked when the unused inner continuation is not present.

The problem only affects let/ec, *not* call/ec:

scheme@(guile-user)> (list 'a (call/ec (lambda (ae) (list 'b (call/ec (lambda (be) (be 2)))))))
$4 = (a (b 2))
scheme@(guile-user)> (list 'a (call/ec (lambda (ae) (list 'b (call/ec (lambda (be) (ae 2)))))))
$5 = (a 2)
scheme@(guile-user)> (list 'a (call/ec (lambda (ae) (list 'b (ae 2)))))
$6 = (a 2)

It also only happens when compiling, not when interpreting:

scheme@(guile-user)> ,o interp #t
scheme@(guile-user)> (list 'a (let/ec ae (list 'b (let/ec be (be 2)))))
$7 = (a (b 2))
scheme@(guile-user)> (list 'a (let/ec ae (list 'b (let/ec be (ae 2)))))
$8 = (a 2)
scheme@(guile-user)> (list 'a (let/ec ae (list 'b (ae 2))))
$9 = (a 2)

-zefram