Use custom solution instead of `strtoull`

[ej-shafran]

Added a macro parse_unsigned_int to be used in functions that will replace the calls to strtoull, very loosely based on the source code for strtoull.

If you think a macro isn't smart here I can inline it to both of the functions, I just felt it's a little repetitive and that it might be useful in case you want to add more numeric types in the future.

An implementation of parse_signed_int should be pretty straightforward