This follows the POSIX-y implementations, using mx_ system calls in
place of mmap, et al.

Some references:
https://fuchsia.googlesource.com/magenta/+/HEAD/docs/objects/vm_address_region.md
https://fuchsia.googlesource.com/magenta/+/HEAD/docs/syscalls/vmo_create.md
https://fuchsia.googlesource.com/magenta/+/HEAD/docs/syscalls/vmar_map.md
https://fuchsia.googlesource.com/magenta/+/HEAD/docs/syscalls/vmar_unmap.md
https://fuchsia.googlesource.com/magenta/+/HEAD/docs/syscalls/vmar_protect.md

Bug: chromium:731217
Change-Id: I7a33c2cc2b41736e395bd3431b88e6b9621b7ca5
Reviewed-on: https://chromium-review.googlesource.com/619687Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47438}