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So, creating interfaces *allocates*. That means `pool.Put([]byte)` allocates,
that's obviously bad. This was, unfortunately, showing up in go-ipfs after
switching to a pool-backed yamux.

What's the solution to too many allocations? More object pools!

They won't use the buffer pool but it doesn't hurt to be nice to the user by
allocating such slices anyways.

Before, this library assumed that we wanted to use pools for things other than
byte slices. In practice, this isn't the case and supporting such use-cases made
this library annoying to use.

This way, copying it won't copy the underlying pool.

Also, initialize it immediately instead of in an `init` function to ensure that
it's fully initialized immediately.

This has moved to libp2p

1. They're small.
2. We can avoid the lock (major speedup).
3. Better memory locality.
4. Much simpler.