Add a MappedROM::find_chunk_starting_with() helper since that's a very
common usage pattern in clients of this code.
Also convert MultiProcessorParser from a persistent singleton object
to a temporary object constructed via a failable factory function.
This patch adds a MappedROM abstraction to the Kernel VM subsystem.
It's basically the read-only byte buffer equivalent of a TypedMapping.
We use this in the ACPI and MP table parsers to scan for interesting
stuff in low memory instead of doing a bunch of address arithmetic.
This was supposed to be the foundation for some kind of pre-kernel
environment, but nobody is working on it right now, so let's move
everything back into the kernel and remove all the confusion.
