stress_stringsu.pir tests encoding via sprintf and concat.
The fixed version is only a bit slower then the old broken version.
13.666s (broken) vs 13.506s (fixed) vs 3.022s (without encoding).
So fixing the encoding representation as STRING* in SREG and consts 'U'
is a worthwile goal.
There is almost no measurable speed advantage from ascii strings to encoded strings.
With the fix and without the fix, as sprintf handling slows down the test 4x times.
So converting a SREG and const to encoded STRING* will not gain much performance.