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."