Split jsonrpc2.go file into multiple files (#65)

This merge request moves some of the contents from the jsonrpc2.go file
into their own designated file. The new files being introduced
(excluding test files) are as follows:

* conn.go
* request.go
* response.go

The motive of this change is to make it easier to navigate the code.
Without this change, the jsonrpc2.go file is 813 lines of code.

internal_test.go

@@ -0,0 +1,35 @@
+package jsonrpc2
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestAnyMessage(t *testing.T) {
+	tests := map[string]struct {
+		request, response, invalid bool
+	}{
+		// Single messages
+		`{}`:                                   {invalid: true},
+		`{"foo":"bar"}`:                        {invalid: true},
+		`{"method":"m"}`:                       {request: true},
+		`{"result":123}`:                       {response: true},
+		`{"result":null}`:                      {response: true},
+		`{"error":{"code":456,"message":"m"}}`: {response: true},
+	}
+	for s, want := range tests {
+		var m anyMessage
+		if err := json.Unmarshal([]byte(s), &m); err != nil {
+			if !want.invalid {
+				t.Errorf("%s: error: %v", s, err)
+			}
+			continue
+		}
+		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)
+		}
+	}
+}