Add TypedArray::copy_from

[MattiasBuelens]

This introduces TypedArray::copy_from(src), as the dual of TypedArray::copy_to(dst).

While this can already be achieved today with TypedArray::set(), it requires either unsafe code or an additional allocation:

let x = [1, 2, 3];
let mut array = Int32Array::new(&3.into());

unsafe { array.set(Int32Array::view(&x), 0); } // view() is unsafe
// or
array.set(Int32Array::from(&x), 0); // from() allocates a new Uint8Array to copy the contents of x

The new helper method provides a safe interface to perform this copy:

let x = [1, 2, 3];
let mut array = Int32Array::new(&3.into());
array.copy_from(&x);

Internally, it uses the unsafe implementation listed above. We know that this is safe, because:

• the TypedArray view on the Rust slice lives only for the duration of the copy_from() call
• set() does not grow the WebAssembly linear memory, so it cannot invalidate the created view

[alexcrichton]

Looks good to me, thanks! I think that the method should probably take &self instead of &mut self as well since it's all JS, but otherwise seems good to go.