Implement span and propagation APIs

[isaachier]

I actually wrote this a few months back for fun. Only thing I don't think I addressed is the tracer itself. Should be a good starting point. Heavily based on opentracing-cpp. Caution: I am no Rust expert, this was written as an attempt to learn the language.

[daschl]

@isaachier thanks much for your contribution! I think this PR mixes a couple things which should be tracked separately.

• there is a separate pull request ongoing which discusses the overall project layout, so I think we should hold off until this one is merged in and then build on top of it
• as a side note, in rust we normally don't check in lock files for libraries, only for binaries
• I'd like to discuss the API further and clarify any concerns before merging something in (see Basic API Discussion #2 for example)

I'd love to keep this open and revisit as we are discussing the individual API elements, and ideally we converge at some point. Does that work for you? I'd love to get your input and review on the other issues/PRs as well if you are interested.