LibWeb: Add a naive Resource cache

This patch introduces a caching mechanism in ResourceLoader. It's keyed
on a LoadRequest object which is what you provide to load_resource()
when you want to load a resource.

We currently never prune the cache, so resources will stay in there
forever. This is obviously not gonna stay that way, but we're just
getting started here. :^)

This should drastically reduce the number of requests when loading
some sites (like Twitter) that reuse the same images over and over.

Libraries/LibWeb/Loader/Resource.cpp

@@ -29,13 +29,13 @@
 
 namespace Web {
 
-NonnullRefPtr<Resource> Resource::create(Badge<ResourceLoader>, const URL& url)
+NonnullRefPtr<Resource> Resource::create(Badge<ResourceLoader>, const LoadRequest& request)
 {
-    return adopt(*new Resource(url));
+    return adopt(*new Resource(request));
 }
 
-Resource::Resource(const URL& url)
-    : m_url(url)
+Resource::Resource(const LoadRequest& request)
+    : m_request(request)
 {
 }