v0.17.1

Fixed

• A str | None field given the text "null" read back as None. The
  optional wire form spells None as the text null, and a present
  string with that exact encoding collided with it; lairs' Token.text
  lost any token spelt null, which Hypothesis found in bead's layers
  round-trip law. A present value whose encoding is null followed by
  any run of backslashes now gains one backslash on the wire and loses it
  on read, so None and every string stay distinct for any inner type.
  No other value changes its stored or pickled form.