DF behaviour: Allow zero-horizon windows/views

[nmnobre]

This aims at supporting zero-horizon windows in OpenStream programs. The behaviour should be semantically equivalent to not referencing the respective stream at all in the task creation statement.

@a-pop commit 46ab217 removes an assertion in wstream_df_resolve_n_dependences(). This allows my programs to run but it will obviously cause problems. In particular, I can no longer use the peek clause, instead having to resort to a zero-burst input.

@a-pop There is an odd zero-decrease call to tdecrease_n() after I modified wstream_df_resolve_dependences() and __built_in_wstream_df_prepare_data so I added a guard to tdecrease_n(). Is this okay? There's probably a better fix once we know the reason for that call.

@Richard549 This bypasses trace state changes on both tdecrease_n() and wstream_df_resolve_dependences() on early exists, can you please check if this is okay?

[nmnobre]

The solution proposed here is, nevertheless, less than ideal. The compiler should produce code to prevent the creation of zero-horizon views altogether in the first place.

[Richard549]

The tracing state transitions are fine, if there are no dependences to resolve then we can avoid swapping in/out of the dependency-resolution state.

(After looking at it, I have also included the code that appends the view to the chain in the state, and included it in the PR.)