[issue7451] improve json decoding performance

[Antoine Pitrou]

Antoine Pitrou <pitrou at free.fr> added the comment:

I've just noticed that there is a persistent decoder instance in
`json._default_decoder`. To avoid keeping references to past keys
forever, this updated patch clears the memo dict when a string has
finished decoding.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue7451>
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