Make the pickle test case run less than 2 seconds.
Added PackWithReplacements method.
Allows injecting exceptions to a thread doing reflective call.

This works by binding the exception (a record) instead of `unit` to the
reflective stream. This assumes we will not raise `unit` as an exception...
Added Serializer.extractByLabels.

This is mainly used to quickly extract Chunks without needing the full
serialized result.