Fix memory leaks in TransformHelper::eval_sliced_Transform() and eval_sliced_Transform().

- The memory leaks were due to a previous work-in-progress optimization reported to later.
- It fixes too much memory consumption failure in pyfaust.demo.bsl (because of Faust normalization using Faust slicing).
- By the way, add a mechanism for this later opt. in Faust::Transform (optional deallocation of data, but only privately accessible to avoid naked access).

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