Populate hashtable of __sleep() properties

Instead of populating a hashtable of property names and then
directly serializing.

This has the advantage of a) detecting duplicate pr

Populate hashtable of __sleep() properties

Instead of populating a hashtable of property names and then
directly serializing.

This has the advantage of a) detecting duplicate properties more
precisely and b) gives us the ability to discard values without
rewriting the serialization string after the fact for GH-5027.

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