Why implement `Buffer` and `Memory`?

Hey I've just been poking around this package and it seems overly complex. Can someone more familiar with it make sure I haven't just missed Chesterton's fence 😛 Happy to work on this if people agree it makes sense

AFAICT, TranscodingStream manages an input and output Buffer; data is read from the input stream into the input Buffer. This is passed to a codec via Memory (which is just an abstraction over the "used" and "free" parts of a Buffer). The codec writes to the output Buffer, and that's given to the user via an output stream when requested.

• Remove Memory. I can't figure out why this is useful, aside from just making things slightly easier for downstream codecs that are C-based and need pointers. If that's the case, we could views instead.
• Remove Buffer. We really don't need our own implementation; I think IOBuffer should work. Fundamentally, we just want to load data from the input stream into some input array, let a codec convert part of that to an output array, and send it to the user via a stream.

Less importantly:

• Remove Error. Why not just try/catch instead of passing yet another variable around as state?

Start by tracing TranscodingStream and the Buffer, Memory, and Error abstractions, then compare their responsibilities with Julia's IOBuffer and the codec interface. The issue does not name files or tests; done would require an agreed design and corresponding updates to the package's buffering and error-handling behavior.