Add the Exchangeable event to the sample' interpreter.
Re-indent.
Use new Exchangeable operation type for crp.
Add node, and dead code, to reg_heap::probability_for_context( ).
Move some code into src/builtins/vector_from_list.H
Add sampling rate paramter for sample' interpreter.
Allow setting the sampling rate.
Pass rate parameter to new_random_modifiable.
Add the rate to the modifiable.