Implement DeferPolicy.lf.

[petervdonovan]

This example program is intended to support an argument I made many months ago that we should implement the simplest possible policy for resolving conflicts between events, with the expectation that more complicated policies can be implemented in library reactors if they are needed.

The problem of conflicting writes to the same port at the same logical time has been compared with the problem of conflicting events scheduled in logical actions. Although ports and logical actions have different implementations, I regard them as "basically the same thing" in the sense that they are nodes in the reaction graph that can be active or inactive, and that thereby determine the activity of reactions that they trigger.

I have argued to uniform use of a simple "overwrite" policy for both situations. In this example, the alternative "defer" policy is implemented inside of a reactor. The example runs correctly using a recent version of the LF compiler.

[edwardalee]

I think this can be done without a type-specific buffer in a way that would make it easy to make it type-agnostic when we support generics. Specifically, you just need a state variable that keeps track of the microstep at which a deferred input should appear on the output. You can then use the (currently internal) schedule function that specifies a microstep (this is used in federated programs).