feat: port S2EdgeCrosser and edge crossing predicates from C++

[HLennart]

hey, i've been using the s2 library for a solid bit but i needed edge crossing, so i ported the c++ version. aside from arbitrary precision floats. the rust version behaves like the c++ (tested by porting the c++ unit tests (unless otherwise noted in the test mods) and running a quick script comparing the outputs of c++ vs. this rust one)

few notes:

• i split it up into s2/edge_crossings/mod.rs and s2/edge_crossings/crosser.rs mainly to somewhat mirror the c++ headers but with a rust module
• for c++ function overloads, i wrote separate chain_* functions
• not using a separate S2CopyingEdgeCrosser, we just copy by default and don't do pointer shenanigans like c++ does
• I'm strongly typing SignedCrossing instead of using -1 0 1 integers (with a conversion function)
• Most of the documentation is straight copied from the c++ implementation and then adapted to the rust specifics

please note the addition of Clone & Copy on Direction so i can store it in a Copy (and Clone) type

fixes #78

[HLennart]

note that it currently links to a crate::s2::contains_vertex_query::ContainsVertexQuery which doesn't exist (yet), which i'd hopefully port aswell

[brawer]

Oooh, nice!!! ❤️