Separate Carrier into Injector and Extractor

[jtescher]

See open-telemetry/opentelemetry-specification#578 for spec changes

[TommyCpp]

I think we can use something like

pub trait Carrier{}

impl<T> Carrier for T where T: Injector + Extractor{}

which basically make Carrier into a marker trait and implement it for every struct that implements Injector and Extractor

[jtescher]

@TommyCpp do you have cases in mind that would prefer to operate on Carrier? It seems likely that they exist, but most of the obvious use cases are either client or server libraries that would probably only impl Injector or Extractor.

[TommyCpp]

@TommyCpp do you have cases in mind that would prefer to operate on Carrier?

Hmmm actually not. I guess it make sense to replace Carrier with Injector and Extractor. I'd like to work on this if that's OK