Add serialize and deserialize methods

[lars-t-hansen]

Lock and Cond have no mutable state and are basically copyable and hence serializable, and we should make it possible to use that capability.

We could perhaps add a serialize instance method that produces a recognizable structure, eg, an object with a field typeIsLock:true or typeIsCond:true and other data fields that are implementation-dependent, and static methods on Lock and Cond that can accept these objects.

Or perhaps the better solution is to hook into the JSON machinery, even though JSON can't represent a SharedArrayBuffer reference.

Either way we want something that can be postMessage'd and restored properly in the receiver.