Relax the assumption that all objects fit in a single block (#3424)
It is possible for the program to allocate single object larger than a block, without going through the normal large-object mechanisms that we have for arrays and threads and so on. The GC was assuming that no object was larger than a block, but #3424 contains a program that breaks the assumption. This patch removes the assumption. The objects in question will still be copied, that is they don't get the normal large-object treatment, but this case is unlikely to occur often in practice. In the future we may improve things by generating code to allocate them as large objects in the first place.