Reorganize propagation code (shrink PR 381) (#444)

* Rename distributedcontext package to correlation

Correlation is the name we agreed upon.

* Move trace propagators to api/trace

The trace propagators tests had to be moved to a testtrace subpackage
to avoid import cycles between api/trace and internal/trace.

Needed to shut up golint about stutter in trace.TraceContext -
TraceContext is a name of a W3C spec, so this stutter is
expected. It's certainly still better than golint's suggestion of
having trace.Context.

* Rename api/propagators to api/propagation

This package will not contain any propagators in the long run, just
the interface definitions.

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

example/basic/main.go

@@ -18,7 +18,7 @@ import (
 	"context"
 	"log"
 
-	"go.opentelemetry.io/otel/api/distributedcontext"
+	"go.opentelemetry.io/otel/api/correlation"
 	"go.opentelemetry.io/otel/api/global"
 	"go.opentelemetry.io/otel/api/key"
 	"go.opentelemetry.io/otel/api/metric"
@@ -82,7 +82,7 @@ func main() {
 
 	ctx := context.Background()
 
-	ctx = distributedcontext.NewContext(ctx,
+	ctx = correlation.NewContext(ctx,
 		fooKey.String("foo1"),
 		barKey.String("bar1"),
 	)
@@ -105,7 +105,7 @@ func main() {
 
 		meter.RecordBatch(
 			// Note: call-site variables added as context Entries:
-			distributedcontext.NewContext(ctx, anotherKey.String("xyz")),
+			correlation.NewContext(ctx, anotherKey.String("xyz")),
 			commonLabels,
 
 			oneMetric.Measurement(1.0),