Add support for merging compressed chunks

Compressed chunks can be merged by applying the same copy+heap swap
technique to the internal compressed heaps as done for the
non-compressed heaps. However, special consideration is necessary for
cases where some chunks are compressed and some are not.

The way this is handled is to pick the first compressed chunk found
among the input chunks, and then use that as the "result" chunk. In
the next step all the chunks' non-compressed heaps are merged followed
by all the "internal" compressed heaps. In the last step, the result
chunk has its non-compressed and compressed heaps swapped with the
merged ones, respectively.

In all other regards, the merging works the same as before when merging
non-compressed chunks.