Pickle support for schema objects

Anyone tried pickling schema objects? I'm getting recursion errors.

Initialisation time seems really expensive for large schemas.

Start by reproducing the recursion error when pickling schema objects and measure initialization time for a large schema. Trace the schema-object initialization and serialization entry points to determine whether these are one problem or two separate changes. Done should include reliable pickle round-tripping and a demonstrated improvement for large-schema initialization.

Scritto dal modello di indicizzazione a partire dal testo della issue.