Add TraceState to SpanContext

[awiede]

Resolves #211

Adds TraceState to SpanContext and gives TraceState new methods per spec.

[jtescher]

Great start @awiede! added a few ideas for potential allocation improvements

[awiede]

@jtescher - I think I addressed all of your comments. RE: using a BTreeMap, I don't think that will work since we aren't sorting on the keys, but instead on the insertion order. Given that this should be a relatively small data set, I updated to just use an ordered collection of tuples. TraceState::get is now O(n) instead O(constant), but given the limitations on the size of tracestate itself (I think the spec says 32 elements), I don't know if that's a big deal.

[jtescher]

@awiede yeah that makes sense I hadn't read the spec all that closely 😬, looked it over and noticed a few other changes we may want to do

[awiede]

@jtescher updated to Fn for insert and delete. Spec technically says that keys should be lowercase which leads to some annoying conflicts with the AWS propagator. I think it's fine for now since the AWS reserved keys only capitalize the first character, custom camel cased keys might get hairy.

[jtescher]

Looks good thanks @awiede