Design document for the new serialzier #288

The http serializer in the burl project are improved versions of those in this project. We need a design document that explains the rationale behind each change.

The document should include (not limited to) changes like:

• Bodies can now be sourced from external buffers without any copies.
• Messages with an unknown length that fit in the internal buffer get a proper Content-Length field before sending.
• Messages with an encoder can skip encoding when the finished payload is smaller than config::enc_thr.
• Small buffers are coalesced before writing to reduce the number of writes.
• Chunked framing now supports trailer headers.
• The config struct is now lean and copied into the serializer, eliminating the need for a shared config object.
• Each message receives its encoder through a polymorphic interface instead of reaching for installed services; the caller is responsible for constructing and supplying the appropriate encoder when needed.