Implement edge crosser

[brawer]

Port the s2 edge crosser (and its data structure, edge crossings) from C++ to Rust. This will be needed to eventually support polylines.

https://github.com/google/s2geometry/blob/master/src/s2/s2edge_crosser.h
https://github.com/google/s2geometry/blob/master/src/s2/s2edge_crosser.cc
https://github.com/google/s2geometry/blob/master/src/s2/s2edge_crosser_test.cc

https://github.com/google/s2geometry/blob/master/src/s2/s2edge_crossings.h
https://github.com/google/s2geometry/blob/master/src/s2/s2edge_crossings.cc
https://github.com/google/s2geometry/blob/master/src/s2/s2edge_crossings_test.cc
https://github.com/google/s2geometry/blob/master/src/s2/s2edge_crossings_internal.h

There might be some code in the Golang port, but I’d recommend looking at C++ because that contains the ground truth for the s2 library, and is better maintained.

[Trinathswain45]

Hi, I’d like to work on this issue. Can you assign it to me?

[brawer]

Have fun, and thanks in advance!

[brawer]

@Trinathswain45 Do you already have anything to share? To review the code, it's actually easier if you split large changes into small chunks. For example, if you have a working version of just edge_crossing with unit tests passing, don’t hesitate to send a pull request with just that alone.

[HLennart]

hi! i ported the c++ version in #103 , would appreciate if you had a look; i would love to port Polygon and Polyline as well (which is blocked by edge crossing)