C: Use uint32_t in utf8.c

[timkaechele]

This PR makes all utf8 apis use uint32_t instead of the more generic int

[marcoroth]

I guess if we tighten up the return types, does it make sense to treat them as uint8_t?

[timkaechele]

I guess if we tighten up the return types, does it make sense to treat them as uint8_t?

I think we won’t gain much in terms of performance.

Using uint32_t for all string lengths may be just a bit more straightforward

[marcoroth]

Fair enough, thank you!