Add PhysicalAddress and VirtualAddress type aliases

[nicholasbishop]

These correspond to EFI_PHYSICAL_ADDRESS and EFI_VIRTUAL_ADDRESS. These usually end up being converted to a pointer type, but notably they are always 64-bit regardless of the target platform. This change revealed a bug in the AllocateType enum; it was using usize for a couple members, but that is not correct for all targets, e.g. i686-unknown-uefi.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits): See the
  Rewriting History guide for
  help.
• Update the changelog (if necessary)

[phip1611]

I double-checked it against the spec, looks good!

PS: I do not understand why UEFI talks about virtual addresses. Due to the identity mapping, all virtual addresses correspond to physical addresses, right? Are there exceptions from that?

[nicholasbishop]

PS: I do not understand why UEFI talks about virtual addresses. Due to the identity mapping, all virtual addresses correspond to physical addresses, right? Are there exceptions from that?

My understanding is that when boot services are active it has the identity mapping. After transitining to runtime-services mode it still has the identity mapping, but the OS probably doesn't want to stay in that mode, so it can call SetVirtualAddressMap to set up a virtual mapping. After that it can continue to use the runtime services with the new mapping.