Add `extend` and `read` to ringbuffer.

[richard-uk1]

I've been using the ringbuffer with its iterators and I can't get the computer to optimize copying, so I made some methods that do it manually. They need examples but I want to check you're interested first.

[mitchmindtree]

Certainly interested, thanks for the PR! Let me know when the examples are in and you'd like me to take a closer look :+1:

[richard-uk1]

After experimentation using copy_from_slice doesn't seem slower than ptr::copy_nonoverlapping and doesn't need unsafe, so I'm using that.

[richard-uk1]

Ready for review.

[richard-uk1]

friendly bump