Add m.censor, adjust m.route.Link to use it (#2538)

Also, restructure a few things for better code reuse.

pathname/assign.js

@@ -1,5 +0,0 @@
-"use strict"
-
-module.exports = Object.assign || function(target, source) {
-	if(source) Object.keys(source).forEach(function(key) { target[key] = source[key] })
-}

pathname/build.js

@@ -1,7 +1,7 @@
 "use strict"
 
 var buildQueryString = require("../querystring/build")
-var assign = require("./assign")
+var assign = require("../util/assign")
 
 // Returns `path` from `template` + `params`
 module.exports = function(template, params) {

pathname/tests/test-assign.js

@@ -1,26 +0,0 @@
-"use strict"
-
-var o = require("../../ospec/ospec")
-
-// force usage of polyfill
-var save = Object.assign
-Object.assign = null
-delete require.cache[require.resolve("../assign")]
-var assign = require("../assign")
-Object.assign = save
-
-o.spec("assign polyfill", function() {
-	o("works", function() {
-		var target = {hello: "world", foo: "bar"}
-		var source = {foo: "foo", extra: true}
-
-		assign(target, source)
-
-		o(target).deepEquals({hello: "world", foo: "foo", extra: true})
-
-		var falsySources = [null, 0, "", false, void 0]
-		falsySources.forEach(function(falsy) { assign(target, falsy) })
-
-		o(target).deepEquals({hello: "world", foo: "foo", extra: true})
-	})
-})