Add a streaming multipart/mixed representation and reader #1587

Part of #1581 (prerequisite of GraphQL incremental delivery), but protocol-neutral.

MultiPartRepresentation buffers or writes a known list of parts. Streaming protocols such as GraphQL incremental delivery need:

• A representation whose parts are produced over time and flushed one by one (like EventStreamRepresentation for SSE), with a final closing boundary.
• A reader that iterates the parts of a multipart/* response as they arrive, with bounded part sizes.
  Tested on its own, independently of GraphQL.