Implement AsRef<JsValue> for Closure<T>

This commit adds an implementation of `AsRef<JsValue>` for the `Closure<T>`
type. Previously this was not possible because the `JsValue` didn't actually
exist until the closure was passed to JS, but the implementation has been
changed to ... something a bit more unconventional. The end result, however, is
that `Closure<T>` now always contains a `JsValue`.

The end result of this work is intended to be a precursor to binding callbacks
in `web-sys` as `JsValue` everywhere but still allowing usage with `Closure<T>`.

Author: alexcrichton

build.rs

@@ -1,2 +1,4 @@
 // Empty `build.rs` so that `[package] links = ...` works in `Cargo.toml`.
-fn main() {}
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+}

crates/cli-support/src/descriptor.rs

@@ -189,13 +189,6 @@ impl Descriptor {
         }
     }
 
-    pub fn ref_closure(&self) -> Option<&Closure> {
-        match *self {
-            Descriptor::Ref(ref s) => s.closure(),
-            _ => None,
-        }
-    }
-
     pub fn closure(&self) -> Option<&Closure> {
         match *self {
             Descriptor::Closure(ref s) => Some(s),