Separate out core/io.c from core/globals.h

_From [brandenj...@gmail.com](https://code.google.com/u/102642871390330622592/) on March 24, 2014 01:00:25_

Move io.c declarations from globals.h into its own header.
Perhaps into its own io library w/ string.c.

This will reduce the complexity of linking io.c into other libraries because of the current globals.h dependency.

Separation:
io.c requires dr_fpu_exception_init and ASSERT_NOT_REACHED at the very least because of iox.h.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1397_