add net and wire struct; move net update into its file

sim/compile.lua

@@ -2,12 +2,6 @@
 local ffi = FFI or require("ffi")
 Simulation = Simulation or {}
 
-ffi.cdef([[
-	struct Net {
-		
-	};
-]])
-
 function Simulation.compile(sim)
 	-- assemble a list of all nets
 	local all_nets = {}