VSpace: Unify the uses of ScratchRegion, ReservedRegion, and MappedRegion

[pittma]

These three types grew at distinct times in the development of VSpace 2: The Address Space Odyssey and they could be contracted into a single type which serves the needs of each use case.

This is a follow up to #9.

[pittma]

If MemoryRegion learns how to unmap itself, we can replace the current uses of ScratchRegion with an exclusive MemoryRegion which gets unmapped after it's been used locally. ELF setup, however, has a special case—it needs pages. Therefore we create a ScratchRegion-like type which aliases the VSpace capability and has ownership of a single page's worth of address space. It uses the ReservedRegion mechanics to set up the intermediate paging structures, then takes MemoryRegions of size PageBits into a temporarily_map function a lá ScratchRegion::temporarily_map_region.