Add various optimizations

This PR contains some optimizations that we use (or plan to start using soon) in production at belena.io. To summarize, these are the main changes included here:

• Add methods to efficiently read and write a single byte at a time
• Avoid allocating memory on Bytes() (at the expense of having a second, pre-allocated buffer)
• Add a faster code path for buffers whose size is a power-of-two.

Check the individual commit messages for details.

For our use case (in which we use a circular buffer on a tight loop), each of these optimizations brought considerable performance improvements.

Very nice PR. Thanks!