Fix for io_test C compilation error.

The error message being displayed by the C compiler (GCC version 4.4.1 or newer) is
meaningful.  It warns that the program is attempting to free something that is not
heap-allocated -- in this case "\n" and the definitions of other ioLiterals.

The reason why the error only appears when --fast is supplied is that aggressive inlining
brings the call to free() into the function where an ioLiteral is passed to it, so under
those circumstances the C compiler can see that something stupid is being attempted.

In the original code, the author thought is was safe to free by explicitly avoiding known
cases where the contents of a c_string was owned by another program entity.  However,
since the IO module also supplies unowned c_strings it is unsafe in general to delete
them.  I added comments liberally to the module code where c_strings are accessed, so
future programmers are warned away from this practice.

Looking forward, except where performance is a concern, strings should be used in
preference to c_strings.  The new implementation of strings will manage memory correctly,
so the programmer who uses such objects need not concern himself with ownership details.