Method _to_char_idx

crates/vm/src/builtins/str.rs:1177–1179

(r: &Wtf8, byte_idx: usize)

    // FIXME: two traversals of str is expensive
    #[inline]
    fn _to_char_idx(r: &Wtf8, byte_idx: usize) -> usize {
        r[..byte_idx].code_points().count()
    }

    #[inline]
    fn _find<F>(&self, args: FindArgs, find: F) -> Option<usize>