futures: bug fix

The problem was in the handing of "lightweight continuations" used to
represent suspended computations, and in particular the handling
of continuation marks (such as the ones added by DrRacket's debugging
mode).

Unfortunately, I wasn't able to construct a simpler test case that
triggers the problem. I think the number of installed continuation marks
has to be just right, and there have to be some replacing marks, and
a lightweight continuaiton has to be captured at the right time
as a result of applying a previously captured continuation.

Closes PR 13427

Merge to v5.3.2