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.

codec_test.go

@@ -0,0 +1,34 @@
+package jsonrpc2
+
+import (
+	"bufio"
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestVarintObjectCodec(t *testing.T) {
+	want := 789
+	var buf bytes.Buffer
+	if err := (VarintObjectCodec{}).WriteObject(&buf, want); err != nil {
+		t.Fatal(err)
+	}
+	var v int
+	if err := (VarintObjectCodec{}).ReadObject(bufio.NewReader(&buf), &v); err != nil {
+		t.Fatal(err)
+	}
+	if want := want; v != want {
+		t.Errorf("got %v, want %v", v, want)
+	}
+}
+
+func TestVSCodeObjectCodec_ReadObject(t *testing.T) {
+	s := "Content-Type: foo\r\nContent-Length: 123\r\n\r\n789"
+	var v int
+	if err := (VSCodeObjectCodec{}).ReadObject(bufio.NewReader(strings.NewReader(s)), &v); err != nil {
+		t.Fatal(err)
+	}
+	if want := 789; v != want {
+		t.Errorf("got %v, want %v", v, want)
+	}
+}