modsys: Add sys.print_exception(exc, file=sys.stdout) function.

The function is modeled after traceback.print_exception(), but unbloated,
and put into existing module to save overhead on adding another module.
Compliant traceback.print_exception() is intended to be implemented in
micropython-lib in terms of sys.print_exception().

This change required refactoring mp_obj_print_exception() to take pfenv_t
interface arguments.

Addresses #751.

py/qstrdefs.h

@@ -403,6 +403,7 @@ Q(version_info)
 #if MICROPY_PY_SYS_MAXSIZE
 Q(maxsize)
 #endif
+Q(print_exception)
 #endif
 
 #if MICROPY_PY_STRUCT