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

Also, restructure a few things for better code reuse.

util/censor.js

@@ -0,0 +1,47 @@
+"use strict"
+
+// Note: this is mildly perf-sensitive.
+//
+// It does *not* use `delete` - dynamic `delete`s usually cause objects to bail
+// out into dictionary mode and just generally cause a bunch of optimization
+// issues within engines.
+//
+// Ideally, I would've preferred to do this, if it weren't for the optimization
+// issues:
+//
+// ```js
+// const hasOwn = require("./hasOwn")
+// const magic = [
+//     "key", "oninit", "oncreate", "onbeforeupdate", "onupdate",
+//     "onbeforeremove", "onremove",
+// ]
+// module.exports = (attrs, extras) => {
+//     const result = Object.assign(Object.create(null), attrs)
+//     for (const key of magic) delete result[key]
+//     if (extras != null) for (const key of extras) delete result[key]
+//     return result
+// }
+// ```
+
+var hasOwn = require("./hasOwn")
+var magic = /^(?:key|oninit|oncreate|onbeforeupdate|onupdate|onbeforeremove|onremove)$/
+
+module.exports = function(attrs, extras) {
+	var result = {}
+
+	if (extras != null) {
+		for (var key in attrs) {
+			if (hasOwn.call(attrs, key) && !magic.test(key) && extras.indexOf(key) < 0) {
+				result[key] = attrs[key]
+			}
+		}
+	} else {
+		for (var key in attrs) {
+			if (hasOwn.call(attrs, key) && !magic.test(key)) {
+				result[key] = attrs[key]
+			}
+		}
+	}
+
+	return result
+}