Implement retain on Windows.

The VirtualAlloc and VirtualFree APIs are different because MEM_DECOMMIT cannot
be used across multiple VirtualAlloc regions.  To properly support decommit,
only allow merge / split within the same region -- this is done by tracking the
"is_head" state of extents and not merging cross-region.

Add a new state is_head (only relevant for retain && !maps_coalesce), which is
true for the first extent in each VirtualAlloc region.  Determine if two extents
can be merged based on the head state, and use serial numbers for sanity checks.