py: Make it so that printing a small int does not allocate heap memory.
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.