v0.16.0

• Added. A schema that refers to itself — a tree whose children are
  trees — is generated. rasuvaeff/openapi-contract 0.13 compiles such a
  schema to $defs plus a local {$ref: '#/$defs/<name>'} for every
  reference back into the cycle; the generator unfolds the def three levels
  below the root and ends at a leaf where the recursive member is left out —
  an optional property omitted, an array that may be empty left empty, a
  branch of anyOf/oneOf skipped, a member of an allOf inlined as the
  def's body. A def whose only way down is a required member that is the def
  itself has no finite instance and is refused (recursive schema "…" has no finite instance), as is a def that is an allOf member of itself; a
  $ref to anything but the schema's own $defs is refused by name. The
  JSON body encoder follows the same references, so a nested empty object
  three levels down is still written as {}. The zoo gains tree.create;
  the recorded corpus grows by its 25 cases.
• Changed. Requires rasuvaeff/openapi-contract ^0.13.