Implement support for Uint8ClampedArray

This commit implements support for binding APIs that take
`Uint8ClampedArray` in JS. This is pretty rare but comes up in a
`web-sys` binding or two, and we're now able to bind these APIs instead
of having to omit the bindings.

The `Uint8ClampedArray` type is bound by using the `Clamped` marker
struct in Rust. For example this is declaring a JS API that takes
`Uint8ClampedArray`:

    use wasm_bindgen::Clamped;

    #[wasm_bindgen]
    extern {
        fn takes_clamped(a: Clamped<&[u8]>);
    }

The `Clamped` type currently only works when wrapping the `&[u8]`, `&mut
[u8]`, and `Vec<u8>` types. Everything else will produce an error at
`wasm-bindgen` time.

Closes #421

src/describe.rs

@@ -3,7 +3,7 @@
 
 #![doc(hidden)]
 
-use JsValue;
+use {JsValue, Clamped};
 
 macro_rules! tys {
     ($($a:ident)*) => (tys! { @ ($($a)*) 0 });
@@ -40,6 +40,7 @@ tys! {
     CHAR
     OPTIONAL
     UNIT
+    CLAMPED
 }
 
 #[inline(always)] // see `interpret.rs` in the the cli-support crate
@@ -202,3 +203,10 @@ impl<T: WasmDescribe> WasmDescribe for Result<T, JsValue> {
         T::describe()
     }
 }
+
+impl<T: WasmDescribe> WasmDescribe for Clamped<T> {
+    fn describe() {
+        inform(CLAMPED);
+        T::describe();
+    }
+}