Refactor propagators (#467)

* Remove binary propagators

They are in process of being dropped from the specification and we
haven't be using them anywhere in the project. Can reintroduce them
later.

* Rename Supplier to HTTPSupplier

The supplier is used only in HTTP propagators currently. It's not
clear if it will be useful for binary propagators if they get to be
specified at some point.

* Rework propagation interfaces

The biggest change here is that HTTP extractors return a new context
with whatever information the propagator is able to retrieve from the
supplier. Such interface does not hardcode any extractor's
functionality (like it was before by explicitly returning a span
context and correlation context) and makes it easy to chain multiple
propagators.

Injection part hasn't changed.

* Add Propagators interface

This interface (and its default implementation) is likely going to be
the propagation API used the most. Single injectors, extractors or
propagators are likely going to be used just as parameters to the
Option functions that configure the Propagators implementation.

* Drop noop propagator

It's rather pointless - just create an empty Propagators instance.

* Fix wrong name in docs

Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>

api/propagation/doc.go

@@ -12,6 +12,5 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// Package propagation contains interface definition for BinaryFormat and
-// TextFormat propagators.
+// Package propagation contains interface definition for HTTP propagators.
 package propagation // import "go.opentelemetry.io/otel/api/propagation"