message: Make the go/core/registry library public.
  It is documented as public:
  And the sample code uses registry directly:
diff: @@ -13,6 +13,7 @@ go_library(
  "registry.go",
  ],
  importpath = "github.com/google/tink/go/core/registry",
  + visibility = ["//visibility:public"],
  deps = [
  "//go/tink:go_default_library",
  "//proto:tink_go_proto",
  