Tcl_GetStringResult() forward compatibility

Direct use of interp->result is now forbidden. An extension that scribbles on interp->result will get pointer smashes. All uses of interp->result have to be replaced with appropriate uses of Tcl_GetStringResult and Tcl_SetResult or their Tcl_Obj counterparts.

In a header file of your C/C++ application or extension that includes the Tcl library, add these lines

#include <tcl.h>
#define Tcl_GetStringResult(interp) (interp->result)

Why? This helps you deprecate direct access to interp->result so you can upgrade your code written for Tcl 7 so it can be used with Tcl 8, yet still work with Tcl 7.