With the implementation of proper string formatting, code to print a
small int was delegated to mpz_as_str_inpl (after first converting the
small int to an mpz using stack memory).  But mpz_as_str_inpl allocates
heap memory to do the conversion, so small ints needed heap memory just
to be printed.

This fix has a separate function to print small ints, which does not
allocate heap, and allocates less stack.

String formatting, printf and pfenv are now large beasts, with some
semi-duplicated code.