soundness(neon): Require T: 'static on JsArrayBuffer::external and JsBuffer::external

[kjvalencik]

Currently, types used as the backing storage of external ArrayBuffer do not need to be 'static! This makes it trivial to create a soundness hole (use after free), with only safe Rust.

pub fn soundness_hole(mut cx: FunctionContext) -> JsResult<JsArrayBuffer> {
    let mut data = vec![0u8, 1, 2, 3];
    
    // Creating an external from `&mut [u8]` instead of `Vec<u8>` since there is a blanket impl
    // of `AsMut<T> for &mut T`
    let buf = JsArrayBuffer::external(&mut cx, data.as_mut_slice());

    // `buf` is still holding a reference to `data`!
    drop(data);

    Ok(buf)
}

Fixes #896