Add pluggable transport interface + WebSocket support

simplify API by using interface{}, rename from transport -> stream

add WebSocket transport in websocket subpackage

do not buffer in ReadObject, rename GetObjectReader/ReadObject -> NextObjectReader

use xtest (jsonrpc2_test) package to allow us to test subpackages that depend on us (in a future change)

factor out vscode-specific transport code and allow pluggable transports

remove Server (unused) and Serve (unnecessary):
  The Serve func had nothing specific to JSON-RPC; it was just a loop
  around (net.Listener).Accept. It added no value.

object_test.go

@@ -0,0 +1,59 @@
+package jsonrpc2
+
+import (
+	"encoding/json"
+	"reflect"
+	"testing"
+)
+
+func TestAnyMessage(t *testing.T) {
+	tests := map[string]struct {
+		request, response bool
+	}{
+		// Single messages
+		`{}`:                                   {},
+		`{"foo":"bar"}`:                        {},
+		`{"method":"m"}`:                       {request: true},
+		`{"result":123}`:                       {response: true},
+		`{"error":{"code":456,"message":"m"}}`: {response: true},
+	}
+	for s, want := range tests {
+		var m anyMessage
+		json.Unmarshal([]byte(s), &m)
+		if (m.request != nil) != want.request {
+			t.Errorf("%s: got request %v, want %v", s, m.request != nil, want.request)
+		}
+		if (m.response != nil) != want.response {
+			t.Errorf("%s: got response %v, want %v", s, m.response != nil, want.response)
+		}
+	}
+}
+
+func TestMessageCodec(t *testing.T) {
+	tests := []struct {
+		v, vempty interface{}
+	}{
+		{
+			v:      &Request{ID: ID{Num: 123}},
+			vempty: &Request{ID: ID{Num: 123}},
+		},
+		{
+			v:      &Response{ID: ID{Num: 123}},
+			vempty: &Response{ID: ID{Num: 123}},
+		},
+	}
+	for _, test := range tests {
+		b, err := json.Marshal(test.v)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if err := json.Unmarshal(b, test.vempty); err != nil {
+			t.Fatal(err)
+		}
+
+		if !reflect.DeepEqual(test.vempty, test.v) {
+			t.Errorf("got %+v, want %+v", test.vempty, test.v)
+		}
+	}
+}