heapblock is a simple `sbrk` style implementation, also useful as an
"endless" decompression buffer. dlmalloc is used on top as a malloc
implementation.
This also changes how the Python side manages its heap. We still use a
python-side malloc implementation (since this is faster), and we put the
Python heap at the m1n1 heap + 128MB, without allocating it.
Hopefully this should never step on anything m1n1 neads, and avoids
having to manage freeing across Python script calls.
Signed-off-by: Hector Martin <marcan@marcan.st>
