11.13 Marker

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().