Refactor VortexFileWriter

[gatesn]

Tracking a bunch of breaking changes to the format and refactoring of the writer

• Consistent naming for buffers / messages. I'm thinking "blocks".
• Ensure we don't write extra padding / framing, e.g. we sometimes use IPCMessage and MessageWriter for writing to the file, even though this adds framing for a streaming format.
• VortexFileWriter to pass chunks into an abstract LayoutWriter.
• Impl LayoutWriter for struct, chunked, flat, and later dict layouts.
• Layout flat buffer to hold BlockIds instead of offset/length. This allows Layout to be abstract over the storage format and not assume linear bytes (e.g. can store in an arbitrary block device).
• Postscript to point to DType + FileLayout and not assume contiguous bytes.