Fix negative score order (backward incompatible!)

[hltbra]

Before this commit, sorted set negative scores were stored after
all positive scores because of their binary representation.
For example:

    >>> struct.pack('>d', -1)
    b'\xbf\xf0\x00\x00\x00\x00\x00\x00'
    >>> struct.pack('>d', +1)
    b'?\xf0\x00\x00\x00\x00\x00\x00'
    >>> struct.pack('>d', -1) < struct.pack('>d', +1)
    False

This commit introduces the FloatCodec class to encode/decode floats into bytes and vice-versa, preserving their numerical order.
For example, the following list is encoded to bytes while keeping and preserves its numerical order:

    >>> floats = [1.5, -2.5, 3.2, 2.0]
    >>> sorted(floats, key=FloatCodec().encode)
    [-2.5, 1.5, 2.0, 3.2]

References:

• https://en.wikipedia.org/wiki/Floating-point_arithmetic#IEEE_754_design_rationale
• https://stackoverflow.com/a/43305015/565999
• https://stackoverflow.com/a/12933766/565999
• https://ananthakumaran.in/2018/08/17/order-preserving-serialization.html
• https://ananthakumaran.in/2018/08/17/order-preserving-serialization.html#float

WARNING: This is a backward incompatible change and requires a major release!

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The bit flip is still magical. It seems to be a well-known way to encode floats/doubles but I couldn't find a reference implementation from the IEEE.