Leave the string stack unchanged, and leave the string body
address and length on the data stack.
NOTE: In earlier versions this was call $S@S. The trailing
"S" is superfluous if it is understood that the only string
format that usually appears on the data stack is the Forth
WARNING: If a$ is a bound string, it may move at the next
garbage collection, making a.str invalid. This can be
avoided by sandwiching sections of code where this could
occur between $GC-OFF and $GC-ON.