The initial design of UnifiedDiffWriter was self-contained,

subclassing io.BytesIO and therefore allowing any standard

operations like seek() or getvalue(). However, this is limiting, as

it means other types of streams (like a file) can be incrementally

written to.
This updates the design to require passing an explicit stream. This

simplifies the design a bit, and ensures a separation between diff

writing and stream operations.
Callers have all been updated to construct an explicit io.BytesIO to

pass to it, and now fetch their results from that method.

All unit tests pass.