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* Move buildRoot into the Action monad, so it can be configured from command line in future

* Move settings from Setting.Path to Base and Context

* Simplify build rule matching and prepare to factoring out common build rules into the library, #347

See #347

* Get rid of unsafePerformIO using shakeExtra
* Move diagnostic info utilities to the library

See #347

See #347

See #347

See #347

See #347

See #347

See #347

See #347

See #347

See #347

See #386

See #384

See #336

See #292, #336

See #387

This brings productivity from roughly 40% to 95%. With parallel GC we generally
spend much of our time synchronizing between the GC threads and relatively
little time doing productive work.