Function shape_pair_key 

pub fn shape_pair_key(s1: i32, s2: i32, c: i32) -> u64

Build a symmetric key from a pair of shape indices and a child index. (b3ShapePairKey)

The smaller shape index goes in the high bits so (a, b, c) and (b, a, c) map to the same key. Layout: [shape_lo : 22][shape_hi : 22][child : 20].