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The format used by the marshal module does not support instancing. This precludes certain data optimizations, such as sharing string constants, common tuples, even common code objects. Since the marshal format is used to write compiled code, this makes it impossible to do data optimization on code prior to writing it out.
This patch adds proper instancing support for all the supported types and increases the default version to three.
(A separate defect/regression is that interned strings are no longer preserved as was implemented in version 1 of the original 2.x branch. This also negates any interning done at compile time. That is a separate defect.)