use a class instead of redefining properties

For the context object passed into handlers, we were previously
redefining the getters every single time, which is slow in V8. This
change adds a class for contexts which holds those getters so they're
only defined once.

js/bootstrap/context.js

@@ -1,4 +1,7 @@
-const { _route } = self._bindings;
+const { _route, getPrivate } = self._bindings;
+const urlSym = getPrivate('url');
+const querySym = getPrivate('query');
+const paramsSym = getPrivate('params');
 
 const REGEX_CAPTURE_GROUPS = /\:([a-zA-Z0-9_]+)/g;
 const REPLACE_CAPTURE_GROUPS = '(?<$1>[^\\/]+)'; // named capture groups
@@ -26,15 +29,21 @@ function parseParamsFromUrlPath(url) {
   return groups;
 }
 
+class Context {
+  constructor(url) {
+    this[urlSym] = url;
+  }
+
+  get query() {
+    return this[paramsSym] || (this[paramsSym] = new URL(this[urlSym]).searchParams);
+  }
+
+  get params() {
+    return this[querySym] || (this[querySym] = parseParamsFromUrlPath(this[urlSym]));
+  }
+
+}
+
 export function generateContextObject(url) {
-  let params;
-  let query;
-  return {
-    get query() {
-      return params || (params = new URL(url).searchParams);
-    },
-    get params() {
-      return query || (query = parseParamsFromUrlPath(url));
-    }
-  };
+  return new Context(url);
 }