[Multipart] Add the frame -> bytes serializer

[czechboy0]

Motivation

Second state machine, doing the inverse of #72, serializes frames into bytes.

Modifications

• A new state machine.
• A new serializer wrapping the state machine.
• An async sequence wrapping the serializer.

Result

We can now serialize multipart frames into bytes.

Test Plan

Unit tests for all 3 layers.

[simonjbeaumont]

Looks good. Minor comment.