remove context public interface

context.go

@@ -7,9 +7,9 @@ import (
 )
 
 // V2rayKey is the key type of Instance in Context, exported for test.
-type V2rayKey int
+type v2rayKeyType int
 
-const v2rayKey V2rayKey = 1
+const v2rayKey v2rayKeyType = 1
 
 // FromContext returns an Instance from the given context, or nil if the context doesn't contain one.
 func FromContext(ctx context.Context) *Instance {