Initial Benchmarks

[contagon]

Does some more rigorous benchmarking. Specifically we add,

• Added clone support for Graph, Values, and Factor using dyn-clone
• Inline a handful of obvious functions
• Added benchmarking crate with rust benchmarks
• Also added C++ package for building and running C++ benchmarks
• Fixed some small covariance bugs in the g2o loader

Still some more libraries that could be benchmarked, but this should be sufficient for now.