Add methods for copying slices of data into and out of the ring buffer

[mgeier]

This has been discussed in #57.

I think the API for writing would be straightforward and the implementation could use the existing write_chunk_uninit() method.

I'm less sure about the API for reading. I guess users could provide an already initialized slice onto which the data will be copied.
However, it might be more efficient in some cases if the user could provide some kind of uninitialized slice.

I think it would be useful to provide those methods for T: Copy types, because under the hood memcopy() or memmove() will be used for maximum efficiency.
I'm not sure if it would also make sense for less constrained types.

[mgeier]

I'm still unsure how to call these methods ... any suggestions?

Since these convenience functions would not break backwards compatibility, I think I'll make a new release soon without them and add them at a later point.

In the meantime, I've (arguably) improved the examples in the docs: #64.

[mgeier]

I've created a possible implementation in #66.