Adds more information to traces with a new Trace' type, which includes the
*sequence* of actions a thread will perform next, and use that for blocking
lookahead. This allows skipping over things like `_concKnowsAbout`, and so
brings the analysis behaviour of `spawn` in-line with `fork`.
For the test cases, this further reduces the average number of runs to 23.
The literature sttes that many bugs are found with 2 concurrent
threads, so encouraging even more parallelism is possibly a win in
only a small number of cases, but potentially a huge cost in many.
This adds a new `MonadConc` primitive, `_concNoTest`, which is (for
all non-test implementations) the identity function. For test
implementations, it is understood as "this action is completely safe
under all schedules, so just execute it all at once and don't consider
any internal interleavings." It is not required to be deterministic,
merely to never fail.
Actions annotated with `_concNoTest` will show up as one step in the
trace, and new `Failure` and `ThreadAction` values have been added.
