`MutBuf::mut_buf` allows access to uninitialized memory.

without necessitating flipping to ByteBuf.

allows code to compile with nightly again.

- 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.