v0.16.1

Added

• sequence_bases_with_context: the k-mer context window as bases (#274).
  #272 moved the signal-level k-mer encoding upstream and leech now calls it
  (rnabioco/leech#222), but one overlap survived, and it was the same shape of
  problem: leech kept its own copy of the context windowing, because
  sequence_ints_with_context returns ints and a training corpus serialises
  the context — sequence_with_kmer_context is a string in the chunk format,
  which data merge/load_chunks read back as one. So the downstream caller
  needed bases, and deriving them from the ints by hand would have been a third
  copy of the window rather than the end of the second.

  The new form is the same cut, padded with UNKNOWN_BASE_CHAR (N), and it
  composes exactly: sequence_to_int of the bases is
  sequence_ints_with_context, padding included, because
  base_to_int(UNKNOWN_BASE_CHAR) is UNKNOWN_BASE. That equivalence is what
  makes this a refactor rather than a new rule, and a test sweeps it over five
  contexts and every offset from the start of the sequence to past its end, so
  a divergence between the two forms fails rather than ships. The windowing
  arithmetic itself is now one private helper (context_range) that both
  public forms call.

  Worth doing because this is the step where KmerContext's halves are not
  interchangeable — swap them and every k-mer is read from a window displaced
  by before - after bases, silently, and encode_signal_kmer cannot detect
  it because it only ever sees their sum. A second copy of precisely that rule
  is the one this crate least wants to keep.