encoder: Fix bool wrt protocol version

Starting from protocol 2 1-byte NEWTRUE/NEWFALSE opcodes are more
efficient compared to 5-bytes e.g. "I01\n.".

It is not only about efficiency, as protocol 4 _forbids_ use of variable
length ASCII-only opcodes - whose data length is determined by doing
forward scan for '\n'.

Without encodeBool changes and only with the tests added it would fail
this way:

    --- FAIL: TestEncode/True/proto=2 (0.00s)
        ogorek_test.go:383: encode:
            have: "\x80\x02I01\n."
            want: "\x80\x02\x88."
    --- FAIL: TestEncode/True/proto=3 (0.00s)
        ogorek_test.go:383: encode:
            have: "\x80\x03I01\n."
            want: "\x80\x03\x88."
    --- FAIL: TestEncode/True/proto=4 (0.00s)
        ogorek_test.go:383: encode:
            have: "\x80\x04I01\n."
            want: "\x80\x04\x88."
    --- FAIL: TestEncode/False/proto=2 (0.00s)
        ogorek_test.go:383: encode:
            have: "\x80\x02I00\n."
            want: "\x80\x02\x89."
    --- FAIL: TestEncode/False/proto=3 (0.00s)
        ogorek_test.go:383: encode:
            have: "\x80\x03I00\n."
            want: "\x80\x03\x89."
    --- FAIL: TestEncode/False/proto=4 (0.00s)
        ogorek_test.go:383: encode:
            have: "\x80\x04I00\n."
            want: "\x80\x04\x89."

encode.go

@@ -211,8 +211,17 @@ func (e *Encoder) encodeArray(arr reflect.Value) error {
 }
 
 func (e *Encoder) encodeBool(b bool) error {
-	var err error
+	// protocol >= 2  ->  NEWTRUE/NEWFALSE
+	if e.config.Protocol >= 2 {
+		op := opNewfalse
+		if b {
+			op = opNewtrue
+		}
+		return e.emit(op)
+	}
 
+	// INT(01 | 00)
+	var err error
 	if b {
 		err = e.emits(opTrue)
 	} else {

ogorek_test.go

@@ -146,6 +146,14 @@ var tests = []TestEntry{
 	X("None", None{},
 		P0_("N.")), // NONE
 
+	X("True", true,
+		P01("I01\n."), // INT 01
+		P2_("\x88.")), // NEWTRUE
+
+	X("False", false,
+		P01("I00\n."), // INT 00
+		P2_("\x89.")), // NEWFALSE
+
 	X("int(5)", int64(5),
 		I("I5\n.")), // INT