We need to duplicate a function for handling space upgrade in read view.
We can't just use func_new(func->def) to do this, because func_new sets
the given func_def to func->def, without copying. Usually, foo_new
duplicates the provided foo_def, e.g. see space_new. Let's make func_new
do the same.

Needed for https://github.com/tarantool/tarantool-ee/issues/163

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