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>

api/correlation/context.go

@@ -0,0 +1,31 @@
+package correlation
+
+import (
+	"context"
+
+	"go.opentelemetry.io/otel/api/core"
+)
+
+type correlationsType struct{}
+
+var correlationsKey = &correlationsType{}
+
+// WithMap enters a Map into a new Context.
+func WithMap(ctx context.Context, m Map) context.Context {
+	return context.WithValue(ctx, correlationsKey, m)
+}
+
+// WithMap enters a key:value set into a new Context.
+func NewContext(ctx context.Context, keyvalues ...core.KeyValue) context.Context {
+	return WithMap(ctx, FromContext(ctx).Apply(MapUpdate{
+		MultiKV: keyvalues,
+	}))
+}
+
+// FromContext gets the current Map from a Context.
+func FromContext(ctx context.Context) Map {
+	if m, ok := ctx.Value(correlationsKey).(Map); ok {
+		return m
+	}
+	return NewEmptyMap()
+}