S2 library not namespaced

First off, thanks for the great library! It has been a joy to use!

My issue is that it was difficult getting S2 to work with my project due to it not being namespaced. Most primitive classes, typedefs, and structs in the S2 library (e.g. `S2Polygon`, `S2CellId`, `S2Point`), as well as some classes with common names in `//utils` (e.g. `Varint`, `Vector3_d`) are declared in the global namespace. These can lead to conflicts, or even if they don't, should generally not be in a global namespace. Similarly for many of the macros and functions in `//base` (especially `//base/port.h`), or for `//base/mutex.h` which appears to collide with any target including its own `absl/synchronization/mutex.h`.

It would be nice if all code was wrapped in `namespace s2` or similar, and all global macros had an `S2_` prefix.