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

crates/cli-support/src/js/js2rust.rs

@@ -159,7 +159,7 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
                 i = i,
                 val = val,
             ));
-            if arg.is_by_ref() {
+            if arg.is_by_ref() || arg.is_clamped_by_ref() {
                 if optional {
                     bail!("optional slices aren't currently supported");
                 }