ModuleInst.place() does not check for overlap

[clavin-xlnx]

#477 has a workaround by adding an overload to place ModuleInst objects without allowing overlapping placement. This should probably be the default behavior. However, the BlockPlacer2 and other code has been working in the opposite sense for some time now. There should be a transition to disallow or make the obvious place() method disallow this by default. One other consideration is that ModuleInst.getAllValidPlacements() uses ModuleInst.place() to identify which locations are valid for placing the module. Usually it does this in a vacuum, but it can also destructively change the Design's set of used SiteInst.