Before this change, we were hard-coding 4 KiB. This meant that systems
with a 16 KiB native page size were wasting 12 KiB per HeapBlock on
nothing, leading to worse locality and more mmap/madvise churn.
We now query the system page size on startup and use that as the
HeapBlock size.
The only downside here is that some of the pointer math for finding the
base of a HeapBlock now has to use a runtime computed value instead of a
compile time constant. But that's a small price to pay for what we get.
Cell::heap() and Cell::vm() needed to access member functions from
HeapBlock, and wanted to be inline, so they were moved to VM.h.
That approach will no longer work with VM.h not being included in every
file (starting from the next commit), so this commit fixes that circular
import issue by introducing secondary base classes to host the
references to Heap and VM, respectively.
