Acumen's imperative reference interpreter

Over the past week Jan has been taking a closer look at Acumen's parallel reference interpreter together with Walid, Corky, Kevin and Adam. Kevin and Adam have been leading the profiling effort, in order to identify potential bottlenecks and to lay down a baseline to measure improvements against. Corky, Walid and Jan have together with Kevin and Adam been looking over the source code to determine if there is an obvious way to start the optimization work, and in the process Jan has come believe that the sequential algorithm that the parallel interpreter defaults to when given a single thread to execute on is in some ways simpler than the pure functional reference interpreter. At some point Jan will look to extract the sequential implementation from the parallel interpreter to have as a potential entry point into the code base for collaborators with an imperative programming background.