Move S2 cell union code to src/geometry/s2_cell_union.rs

[nnmm]

This is to make src/math.rs less of a grab-bag. I think having this here makes sense because an S2 cell union is one of the query types, like Aabb, Obb and Frustum, and it is a geometric construct.

[nnmm]

ptal @catevita