Function xyz_to_face_si_ti 

pub fn xyz_to_face_si_ti(p: &S2Point) -> (u8, u8, u32, u32)

Convert a direction vector (not necessarily unit length) to (face, si, ti) coordinates and, if p is exactly equal to the center of a cell, return the level of this cell (31 otherwise as its outside the bounds of levels). Return (face, zoom, si, ti).