This adds an additional layer (ChipTop) between the System module and the TestHarness. The IOBinder API is now changed to take only a single parameter (an Any) and return a 3 things: The IO port(s), the IO cell(s), and a function to call inside the test harness, which is analogous to the old IOBinder function, except that it takes a TestHarness object as an argument instead of (clock, reset, success).
* A new Top-level module, ChipTop, has been created. ChipTop instantiates a "system" module specified by BuildSystem.
* BuildTop now builds a ChipTop dut module in the TestHarness by default
* A new BuildSystem key has been added, which by default builds DigitalTop (previously just called Top)
* The IOBinders API has changed. IOBinders are now called inside of ChipTop and return a tuple3 of (IO ports, IO cells, harness functions). The harness functions are now called inside the TestHarness (this is analogous to the previous IOBinder functions).
* IO cell models have been included in ChipTop. These can be replaced with real IO cells for tapeout, or used as-is for simulation.
* The default for the TOP make variable is now ChipTop (was Top)
