Markers are allocated in frob blocks, as usual. They are kept
in a buffer unordered, but in a doubly-linked list so that they
can easily be removed. (Formerly this was a singly-linked list,
but in some cases garbage collection took an extraordinarily
long time due to the O(N^2) time required to remove lots of
markers from a buffer.) Markers are removed from a buffer in
the finalize stage, in ADDITIONAL_FREE_marker().