Improvement of integer to text conversion

[Phidias618]

I replaced the previous algorithm that converts integers to ASCII with a much faster one, for example the worst case of this new algorithm is approximately twice as fast as the best case of the previous algorithm.
As an added benefit, the lower the digit count, the faster the algorithm is done.

This algorithm don't require the use of .itoa_fmt_buf, which allows to save some RAM.

I made a few changes to function signatures

• .utoa_fmt doesn't read __itoa_fmt_len from RAM, it receives it from A instead
• itoa_fmt follows the __sdcccall(1) calling convention instead of OLDCALL
• itoa_fmt returns the pointer to the end of the new string instead of returning its length, this allows to get rid of some 16 bit math both in itoa_fmt and in load_text