Immutability considerations

[ecpeterson]

Almost everywhere in the blossom algorithm, we are careful not to replicate references to objects (especially among different processes!) and instead make copies of the value to share and edit. However, this is not actually enforced, which surely has the potential to lead to unexpected headaches. Consider enforcing immutability so that it is wholly safe to accidentally share objects.

This is a double-edged sword, as it may make implementation / compilation trickier on real hardware, which is likely to be space-limited.

See also #12.