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/convert/impls.rs

@@ -4,7 +4,7 @@ use core::mem::{self, ManuallyDrop};
 use convert::{Stack, FromWasmAbi, IntoWasmAbi, RefFromWasmAbi};
 use convert::{OptionIntoWasmAbi, OptionFromWasmAbi, ReturnWasmAbi};
 use convert::traits::WasmAbi;
-use JsValue;
+use {JsValue, Clamped};
 
 unsafe impl WasmAbi for () {}
 
@@ -373,6 +373,22 @@ impl<T: OptionFromWasmAbi> FromWasmAbi for Option<T> {
     }
 }
 
+impl<T: IntoWasmAbi> IntoWasmAbi for Clamped<T> {
+    type Abi = T::Abi;
+
+    fn into_abi(self, extra: &mut Stack) -> Self::Abi {
+        self.0.into_abi(extra)
+    }
+}
+
+impl<T: FromWasmAbi> FromWasmAbi for Clamped<T> {
+    type Abi = T::Abi;
+
+    unsafe fn from_abi(js: T::Abi, extra: &mut Stack) -> Self {
+        Clamped(T::from_abi(js, extra))
+    }
+}
+
 impl IntoWasmAbi for () {
     type Abi = ();