Bind g_utf8 / g_unichar APIs

[sdroege]

std doesn't have almost anything of that, and the crate implementing all that seems is using icu, which is huge and annoying to build. The utf8 APIs should probably take &GStr / GString, otherwise we end up creating lots of copies.

[bilelmoussaoui]

Should this be closed now? cc @sdroege

[sdroege]

There's still API missing, like g_utf8_next_char()

[RealKC]

When binding the g_utf8 APIs I didn't include the ones that I thought were already offered by the rust stdlib through str (which GStr gets through its Deref<Target = &str> impl), IIRC that was one of them.

However, the g_unichar APIs are still missing (some of them are offered by the stdlib already, but not all), so this issue should stay open for the time being.