#873 change _utoa, _itoa, _ultoa and _ltoa function signature

[Pedro de Medeiros]

_itoa, _uitoa, _ltoa and _ultoa functions should change signature to return a char* instead of void, since it's easier and faster to change the pointer of the beginning of the string buffer than to move the contents of the string. It's also the default implementation of _itoa and _uitoa in several other platforms.

[Pedro de Medeiros]

For instance: https://cplusplus.com/reference/cstdlib/itoa/

[Ronaldo Prado]

Yes. I think this improvment will turn thes functions more useful.