fhdl: support len() on signals

migen/fhdl/structure.py

@@ -157,6 +157,9 @@ def __init__(self, bv=BV(), name=None, variable=False, reset=0, name_override=No
 		self.reset = Constant(reset, bv)
 		self.name_override = name_override
 		self.backtrace = tracer.trace_back(name)
+
+	def __len__(self):
+		return self.bv.width
 
 	def __hash__(self):
 		return id(self)