This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.
Let's just not support building that way, so String.h can have an
objectively nicer name. :^)
This is in preparation for eventually using it in userspace.
LinearAddress.h has not been moved for the time being (as it seems to be
only used by a very small part of the code).
