Marshaler should be extensible

[lars-t-hansen]

The inspiration here is that it is interesting to send objects that represent both client data types and parlib-provided shared-memory types. Imagine creating an AtomicBox (Issue #18) on one agent and then just sending it to another, without having to handle the creation on the receiving agent.

[lars-t-hansen]

Other aspects of extensibility are:

• filtering of keys and values
• a ceiling on the amount of data that will be transmitted, either in terms of the number of values that will be marshaled or the number of marshaled words that will be allowed

[lars-t-hansen]

[master 1a403f3] implements the toMarshaled() protocol and makes the Marshaler reliably reentrant.

Filtering and data ceiling will be recorded elsewhere.