This is more efficient and works most of the time. Additionally,
we can potentially join with a previous free chunk in the allocator,
avoiding extra fragmentation on the heap.
app_server (on x86_64) only has 1 "rld heap" area after this change
with a size of 0x50000 (320KB), whereas previously it had around 7
with a total size of 0x80000 (512KB).
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