[issue19256] Optimize marshal format and add version token.

[Antoine Pitrou]

Antoine Pitrou added the comment:

I actually agree with Kristjan that an opcode is the least disruptive
choice here. I also agree it's useful if we want to be able to evolve
the protocol without being tied by backwards compatibility constraints.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19256>
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