Support a "clocked_by" syntax in BH module instantiations

[quark17]

In BSV, the default clock and reset for a module instantiate can be specified using clocked_by and reset_by clauses. BH doesn't have this, but since the BSV syntax just desugars to a primitive function call, you can manually write that call in BH and get the same behavior. Specifically, the BSV parser translates these into a call to changeSpecialWires, which takes maybe a clock and maybe a reset as arguments (and also maybe a power line, but that's unused infrastructure for a potential future). I guess this isn't too ugly, but we might consider whether a simpler syntax is possible.