as_bytes_mut is missing for ArrayBuffer

[stevefan1999-personal]

    pub fn as_bytes_mut(&mut self) -> Option<&mut [u8]> {
        let (len, ptr) = Self::get_raw(self.as_value())?;
        Some(unsafe { slice::from_raw_parts_mut(ptr, len) })
    }

This is needed to implement TextEncoder

[DelSkayn]

The provided code would allow you to have aliasing mutable references to the same slice, by cloning the ArrayBuffer object and then calling that method on both objects. This would be unsound. I will see if I can provide a mutable way to access the bytes but it would require implementing some RefCell like functionality which is currently not present.

[DelSkayn]

Closing this issue as supporting the feature would be unsound.