Pass a state through the continuations in all cases

[harendra-kumar]

Currently we pass the SVar state only when needed. SVar is a specific state that we need in specific cases. However we need a generalized state passing for configuration type combinators to work. The SVar state could be a subset of the general state that is always passed.

We may need combinators like the following, that can be implemented using a general state passing:

• trace - to trace the threads created and deleted by the stream
• buffer - to specify the size of the buffer to be used on producer side
• threads - to specify a limit of the worker threads on the producer side

Other than implementing the combinators using the internal state passing, as an experimental thought, the same state passing functionality can be used to implement a state monad functionality in streamly itself. Even though a StateT can be used, it may be more efficient and beginner friendly to have integrated get/put.

[harendra-kumar]

For tracing some useful functionality can be:

• Diagnostics - for each SVar print how much time an action is taking on
  average and then suggest whether the parallelism is useful or not, so that the
  user can use appropriate combinators. it should print the source line, the
  Streamly API name and the timings.
• Print total number of SVars and threads and threads per SVar etc under a
  given tree.

[harendra-kumar]

This is fixed via commit 211df79. New issues raised for tracing and State monad integration.