- This fixes#25 and fixes#26.
- Move the mark state into a 'Mark' enum.
- Store both 'pos' and 'len' inside the mark because both
are needed to distinguish between full/empty states. This
fixes 'reset' setting 'len' to zero when the buffer was
full when marked.
- Clear the mark when the length of the marked input range
exceeds the capacity. Fixes bugs with the mark not being
cleared correctly on writes.
- Add test cases.
Carl Lerche