Implement the Propagation API

This ticket is to track updating the propagation API to match the spec: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/api-propagators.md

See [otep 66](https://github.com/open-telemetry/oteps/blob/master/text/0066-separate-context-propagation.md) for more details.