Move mtxio.* to fileio/

This removes some cyclic module-level dependencies, and is more in line
with the current layout, where all I/O code specific to some file
formats is in fileio/.  math/ in general probably also needs to be lower
in the dependency stack than fileio/, so it is not possible to keep the
code in the old place without introducing a cyclic dependency.

Change-Id: If0c0632a3d5b3cd4ffe82f38f743e9afdc7ffdff