ent little-endian binary string.

    >>> decode_long(b'')
    0
    >>> decode_long(b"\xff\x00")
    255
    >>> decode_long(b"\xff\x7f")
    32767
    >>> decode_long(b"\x00\xff")
    -256
    >>> decode_long(b"\x00\x80")
    -32768
    >>> decode_long(b"\x80")
    -128
    >>> decode_long(b"\x7f")
    127
    rē