Make unrooted_must_root a bit more aggressive.

Basically, instead of trying to check for specific kinds of statements,
just check the types of all local variables.

Also included are some commented-out proposals for some slightly more
aggressive lints which might be useful (but trigger a little too
frequently at the moment).

components/script/dom/webglrenderingcontext.rs

@@ -109,6 +109,7 @@ impl WebGLRenderingContext {
         })
     }
 
+    #[allow(unrooted_must_root)]
     pub fn new(global: GlobalRef, canvas: &HTMLCanvasElement, size: Size2D<i32>, attrs: GLContextAttributes)
                -> Option<Root<WebGLRenderingContext>> {
         match WebGLRenderingContext::new_inherited(global, canvas, size, attrs) {