Move serialization import into network implementation

The (default) network implementation is the only place where
`serialization` is used.  Thus, move the import into its constructor so
we don't unnecessarily load the module unconditionally.

bc.lua

@@ -1,4 +1,3 @@
-local serialization = require("serialization")
 local uuid = require("uuid")
 local computer = require("computer")
 local bit32 = require("bit32")
@@ -46,6 +45,7 @@ function Network:new(modem, port)
     modem = modem or require("component").modem,
     port = port or Network.default_port,
     event = require("event"),
+    serialization = require("serialization"),
   }
   setmetatable(self, {__index=Network})
 
@@ -59,17 +59,17 @@ function Network:start(callback)
     -- Filter everything we don't care about
     if addr_lo ~= self.modem.address then return end
     if port ~= self.port then return end
-    callback(addr_remote, serialization.unserialize(msg))
+    callback(addr_remote, self.serialization.unserialize(msg))
   end
   self.event.listen("modem_message", self.listener)
 end
 
 function Network:send(addr, msg)
-  self.modem.send(addr, self.port, serialization.serialize(msg))
+  self.modem.send(addr, self.port, self.serialization.serialize(msg))
 end
 
 function Network:broadcast(msg)
-  self.modem.broadcast(self.port, serialization.serialize(msg))
+  self.modem.broadcast(self.port, self.serialization.serialize(msg))
 end
 
 function Network:pull(filter, timeout)
@@ -78,7 +78,7 @@ function Network:pull(filter, timeout)
     if ev ~= "modem_message" then return false end
     if addr_lo ~= self.modem.address then return false end
     if port ~= self.port then return false end
-    last_msg = serialization.unserialize(msg)
+    last_msg = self.serialization.unserialize(msg)
     return filter(addr_remote, last_msg)
   end)
   if ev ~= nil then